Circadian Rhythms

Circadian Rhythms

A circadian rhythm is an endogenous oscillation with a period near twenty-four hours. It persists without environmental time cues and can be reset by them. The consumer sentence treats “circadian” as a brand for any daily habit. This article keeps the word for the clocks — the suprachiasmatic nucleus, the molecular loop that runs in most cells, the light that moves the central pacemaker, and the meals that can move the liver without moving the brain. It then asks which health claims and which interventions survive that vocabulary. It is a research review. It is not medical advice.

Compiled by South Beach Longevity · 20 August 2026 Copyright 2026 Series SBL-41 / SP-CIRCADIAN-RHYTHMS · Register A scientific article Sources peer-reviewed clock biology, human laboratory protocols, occupational epidemiology, and labelled intervention trials · verified NCBI records Constraint This document describes published research. It is not medical advice. No human use, dose, route or schedule is recommended anywhere in this document.

How to read this document Every finding is labelled, in the sentence that reports it, by the kind of study that produced it. A mouse knockout is not a patient. A forced-desynchrony protocol is not a night-shift career. An occupational hazard ratio is not a mechanism. A short isocaloric feeding window is not a diet. Amounts and clock times appear only as reported study parameters, always with the population attached. Nothing here is a recommendation. Sibling title SBL-41/SP-FASTING owns calorie-matched fasting trials; this title owns clocks, light, and timing as an independent variable.

01 The word that must not mean a habit

Pittendrigh and Aschoff built the modern vocabulary before anyone sold a sunrise lamp. A rhythm is circadian only if it is generated from within, has a free-running period near twenty-four hours, and is entrainable by a zeitgeber (Pittendrigh, 1960; Aschoff, 1965). A daily habit that disappears in constant conditions is not a clock. It is a schedule.

Human isolation and forced-desynchrony work established the endogenous fact. When light–dark and sleep–wake are desynchronized from the internal day, melatonin, cortisol, and core temperature continue to oscillate with a period that is close to, and usually slightly longer than, twenty-four hours (Czeisler et al., 1999; Wright, Hughes, Hull, and Czeisler, 2001; Duffy and Czeisler, 2002). That period is a property of the pacemaker. It is not a moral preference and not a productivity tip.

This article uses circadian only for that class of oscillator. Diurnal means occurring in the day. Nycthemeral means a twenty-four-hour pattern that may be imposed. Misalignment means a measured phase angle between clocks, or between clocks and the social day — not a feeling of being “off.” Chronotype is a phase-of-entrainment phenotype, usually operationalized as a questionnaire or as mid-sleep on free days. It is not a virtue.

02 The suprachiasmatic nucleus

The master pacemaker in mammals sits in the anterior hypothalamus, immediately above the optic chiasm. Bilateral lesions of the suprachiasmatic nucleus (SCN) abolish coherent adrenal corticosterone and drinking/activity rhythms in rodents (Moore and Eichler, 1972; Stephan and Zucker, 1972). Those lesion studies are the necessity proof. They are not a human neurosurgical series.

The sufficiency proof is a transplant. Ralph, Foster, Davis, and Menaker restored rhythmicity in arrhythmic hosts with fetal SCN grafts and showed that the restored period followed the donor, including the short-period tau mutant hamster (Ralph, Foster, Davis, and Menaker, 1990). Period is therefore a tissue-autonomous property of SCN cells, not a property of the body that receives them.

The SCN receives a direct retinal projection — the retinohypothalamic tract — and thereby learns the solar day. It does not need image-forming vision to do that job. Outputs run through hypothalamic and brainstem relays to pineal melatonin, the autonomic nervous system, sleep–wake circuitry, and, indirectly, to adrenal and other endocrine axes (Mohawk, Green, and Takahashi, 2012). Destroy the SCN and the body does not become timeless at the cellular level. Peripheral clocks continue. What is lost is coherence: a shared phase that can be read as one animal’s day.

Human SCN anatomy is inferred from homology, from the same retinohypothalamic logic, and from the clinical observation that hypothalamic injury can fragment sleep–wake timing. The lesion-and-transplant logic remains rodent. That bound is kept in every later sentence that treats the SCN as “the” human clock.

03 The molecular loop: CLOCK/BMAL1 and PER/CRY

Most mammalian cells keep time with a transcription–translation feedback loop. The basic helix-loop-helix PAS transcription factors CLOCK and BMAL1 (ARNTL) heterodimerize, bind E-box elements, and activate the Period (Per1, Per2, Per3) and Cryptochrome (Cry1, Cry2) genes. PER and CRY proteins accumulate, enter the nucleus, and repress CLOCK:BMAL1, closing the loop on a near-twenty-four-hour timescale (Gekakis et al., 1998; Kume et al., 1999; Shearman et al., 2000; Partch, Green, and Takahashi, 2014; Takahashi, 2017). A second loop, through REV-ERB and ROR nuclear receptors, stabilizes Bmal1 transcription. Casein kinase 1 and ubiquitin ligases set the delay. The biochemistry is a delay line, not a metaphor.

The genetics that made the loop visible are mouse genetics. Vitaterna and colleagues recovered a long-period, then arrhythmic, Clock mutant from an ENU screen (Vitaterna et al., 1994). King and colleagues identified the gene (King et al., 1997). Bunger and colleagues showed that Mop3/Bmal1 knockout mice lose behavioural circadian rhythmicity (Bunger et al., 2000). Those papers prove that named genes are necessary for the organismal rhythm in the mouse. They do not prove that a supplement “supports CLOCK/BMAL1” in a reader.

Human Mendelian findings sit beside the mouse, not on top of it. Familial advanced and delayed sleep-phase syndromes have been mapped to PER2, CSNK1D, CRY1, and related loci in small kindreds (reviewed in Patke, Young, and Axelrod, 2020). A mutation that advances sleep in a family is evidence that the same loop runs in people. It is not a population chronotype test and not a licence to moralize evening types.

04 Peripheral clocks

In 1998, Balsalobre, Damiola, and Schibler showed that a serum shock starts a circadian gene-expression oscillation in cultured fibroblasts (Balsalobre, Damiola, and Schibler, 1998). The clock is therefore not a privilege of the SCN. Yamazaki and colleagues then watched transgenic rats with a Per1-luciferase reporter and showed that peripheral tissues oscillate and can be reset, with the SCN recovering faster from a large shift than liver or lung (Yamazaki et al., 2000). Yoo and colleagues, with a PER2::LUC knock-in, found persistent luminescence rhythms in explants from most organs (Yoo et al., 2004).

Feeding is the peripheral zeitgeber that matters for this title. Restricted feeding can invert the phase of liver clocks while the SCN, still locked to the light–dark cycle, holds its phase (Damiola et al., 2000; Stokkan, Yamazaki, Tei, Sakaki, and Menaker, 2001). Food-anticipatory activity can survive SCN lesions, which is the operational definition of a food-entrainable oscillator whose anatomical home is still disputed (Mistlberger, 1994; Honma and Honma, 2009). The sentence that follows is the one consumer circadian talk usually refuses: a meal can time the liver without timing the brain.

That uncoupling is the biological content of “internal misalignment.” It is not a feeling. It is two oscillators reporting two days.

05 Zeitgebers

A zeitgeber is a synchronizing cue, not a wellness product. For the SCN, light is first. For the liver, food is first. Temperature cycles, social contact, and exercise can shift clocks, but in humans they are weaker than light for the central pacemaker and weaker than feeding for the liver (Dibner, Schibler, and Albrecht, 2010; Mohawk, Green, and Takahashi, 2012). Calling caffeine, cold plunges, or a morning journal a zeitgeber without a phase measurement is branding.

Entrainment has a range. A zeitgeber that is too weak, or timed to the wrong part of the phase-response curve, does not lock the clock. It jitters it. The PRC is therefore not an appendix. It is the definition of a timed stimulus (see §13).

06 Clock-biology matrix

07 Light and the melanopsin pathway

Image-forming vision is not the circadian photoreceptor. Provencio and colleagues identified melanopsin in the inner retina (Provencio et al., 2000). Berson, Dunn, and Takao recorded intrinsically photosensitive retinal ganglion cells (ipRGCs) that depolarize to light without rods or cones (Berson, Dunn, and Takao, 2002). Hattar and colleagues and Panda and colleagues placed melanopsin in those cells and tied it to non-image photic responses (Hattar et al., 2002; Panda et al., 2002). The action spectra for human melatonin suppression peak in the short-wavelength, blue-appearing band near 460–480 nm (Brainard et al., 2001; Thapan, Arendt, and Skene, 2001). That fact is real. It is also the seed of a marketing industry (see §38).

Human sensitivity is high. Zeitzer, Dijk, Kronauer, Brown, and Czeisler mapped a steep melatonin-suppression dose–response to ordinary indoor illuminances in a controlled laboratory protocol (Zeitzer, Dijk, Kronauer, Brown, and Czeisler, 2000). Lewy, Wehr, Goodwin, Newsome, and Markey had already shown that bright light suppresses human melatonin (Lewy, Wehr, Goodwin, Newsome, and Markey, 1980). Gooley and colleagues later showed that ordinary room light before bedtime shortens melatonin duration in a home-like laboratory setting (Gooley et al., 2011). Chang, Aeschbach, Duffy, and Czeisler compared a light-emitting eReader with a printed book in the hours before sleep: the eReader suppressed evening melatonin, delayed the dim-light melatonin onset, reduced evening sleepiness, and impaired next-morning alertness (Chang, Aeschbach, Duffy, and Czeisler, 2015). Those are laboratory and controlled-housing results. They are not a proof that one telephone application controls disease.

Lucas and colleagues argued that photometry for circadian and other non-visual responses must be melanopic, not merely photopic lux (Lucas et al., 2014). Brown and colleagues later published expert recommendations that separate bright melanopic daytime exposure from much dimmer evenings (Brown et al., 2022). Recommendations are consensus, not outcomes. They are cited here as a measurement standard, not as a healthspan trial.

08 Melatonin

Pineal melatonin is an SCN-timed darkness signal. In an entrained person it rises in the evening (the dim-light melatonin onset, DLMO), stays high in the dark, and is low in the day (Lewy and Sack, 1989; Arendt, 1998). Light at night suppresses it. Darkness permits it. It is not, by itself, “the sleep hormone.” People can sleep with a suppressed melatonin profile, and melatonin can shift phase without being a potent hypnotic.

Exogenous melatonin is a different object: a timed drug-like signal with a phase-response curve that is, roughly, opposite in sign to the light PRC (Lewy et al., 1998; Burgess, Revell, Molina, and Eastman, 2010; St Hilaire et al., 2012). Meta-analyses of melatonin for primary insomnia report modest reductions in sleep latency, not a restoration of slow-wave architecture (Brzezinski et al., 2005; Buscemi et al., 2005; Ferracioli-Oda, Qawasmi, and Bloch, 2013). The American Academy of Sleep Medicine’s 2017 insomnia guideline gave melatonin a weak or no recommendation for several adult insomnia uses (Sateia, Buysse, Krystal, Neubauer, and Heald, 2017). Jet-lag and some circadian rhythm sleep–wake disorder indications are a separate literature (Herxheimer and Petrie, 2002; Morgenthaler et al., 2007; Auger et al., 2015). Those papers are reviewed as chronobiology, not as a dose table. No amount is recommended here.

09 Cortisol

The cortisol rhythm is an SCN-coordinated endocrine day: low around habitual sleep onset, rising through the night, peaking near habitual wake (Weitzman et al., 1971). It is an output, not a zeitgeber of first rank. A flattened or shifted cortisol curve can mark night work, jet lag, depression, or illness. It can also mark assay timing and awakening. Treating a single morning cortisol as “adrenal circadian health” is a clinic-marketing error.

Forced desynchrony and simulated night-shift protocols can dissociate cortisol from the sleep–wake cycle and from meals (Scheer, Hilton, Mantzoros, and Shea, 2009; Morris, Yang, and Scheer, 2012). That dissociation is a laboratory fact. It is not a diagnosis.

10 Body temperature

Core body temperature, in an entrained human, falls through the evening, reaches a nadir in the second half of the night, and rises toward wake (Czeisler, Weitzman, Moore-Ede, Zimmerman, and Knauer, 1980; Kräuchi, 2007). Distal skin warming and the subsequent core drop are tightly linked to sleep-onset propensity (Kräuchi, Cajochen, Werth, and Wirz-Justice, 1999). The temperature rhythm is one of the most robust human circadian markers still measurable without melatonin assays. It is also slow and easily masked by activity, posture, and meals. A wearable “readiness” score is not a constant-routine temperature protocol.

11 Sleep propensity

Borbély’s two-process model remains the least bad public sentence about sleep timing: a homeostatic Process S rises during wake and falls during sleep; a circadian Process C opens and closes a gate (Borbély, 1982; Daan, Beersma, and Borbély, 1984). Forced-desynchrony work showed that the circadian system produces a wake-maintenance zone in the evening and a critical zone of sleepiness near the core-temperature minimum, even when time awake is held constant (Dijk and Czeisler, 1994; Dijk and Czeisler, 1995). Lavie described an evening “forbidden zone” for sleep (Lavie, 1986).

Sleep propensity is therefore not a function of clock time on the wall. It is a function of the angle between Process S and Process C. A person who has been awake since 04:00 and a person who woke at 09:00 do not occupy the same gate at 22:00. Chronotype moves the gate. Sleep debt fills S. Light moves C. Collapsing those into “everyone should sleep at 22:00” is the error §37 exists to refuse.

12 Chronotype

Horne and Östberg’s morningness–eveningness questionnaire is a preference instrument (Horne and Östberg, 1976). Roenneberg’s Munich ChronoType Questionnaire estimates phase of entrainment from mid-sleep on free days, corrected for sleep debt (Roenneberg, Wirz-Justice, and Merrow, 2003; Roenneberg et al., 2007). The two instruments correlate and are not identical. Chronotype distributes continuously. It late-shifts through adolescence and early adulthood and earlier-shifts with age (Roenneberg et al., 2004; Fischer, Lombardi, Marucci-Wellman, and Roenneberg, 2017).

A late chronotype is a phase. It is not laziness, and an early chronotype is not virtue. Social schedules built for early types manufacture a discrepancy that Wittmann named social jet lag (Wittmann, Dinich, Merrow, and Roenneberg, 2006). Genetics can produce extreme phases; most of the population variance is smaller and mixed with light history, age, and alarm clocks (Patke, Young, and Axelrod, 2020). Treating a 22-year-old evening type as a failed 55-year-old morning type is a category error.

13 Phase-response curves

A phase-response curve plots the shift a stimulus produces as a function of the internal time at which it is given. For light in humans, evening and early-night exposure delays the pacemaker; late-night and morning exposure advances it; the crossover sits near the core-temperature minimum (Minors, Waterhouse, and Wirz-Justice, 1991; Khalsa, Jewett, Cajochen, and Czeisler, 2003; St Hilaire et al., 2012). Melatonin’s PRC is roughly inverted: evening melatonin advances; morning melatonin delays (Lewy, Ahmed, Jackson, and Sack, 1992; Burgess et al., 2010).

Without a PRC, “morning light” and “evening melatonin” are slogans. With a PRC, they are hypotheses about a named phase. The same 08:00 light that advances an early type can sit nearer the delay zone of a delayed type who woke at noon. Universal clock-time prescriptions ignore that geometry.

14 Social jet lag

Social jet lag is the hour-angle between mid-sleep on work days and mid-sleep on free days (Wittmann, Dinich, Merrow, and Roenneberg, 2006). It is a schedule discrepancy, not a diagnosis and not a jet-lag protocol. Roenneberg, Allebrandt, Merrow, and Vetter reported that larger social jet lag tracked higher body-mass index in a large questionnaire sample (Roenneberg, Allebrandt, Merrow, and Vetter, 2012). That is an association in self-reported sleep and self-reported weight. It is not a randomized assignment to a Monday.

Social jet lag co-travels with short weekday sleep, evening light, alcohol, and later eating. Any one of those could carry the metabolic association. The construct is still useful: it names the collision between biological phase and social time without pretending the collision is a character flaw.

15 Light matrix

16 Metabolism

Laboratory circadian misalignment is a metabolic intervention. Scheer, Hilton, Mantzoros, and Shea placed healthy adults on a 28-hour behavioural day so that eating and sleeping occurred 12 hours out of phase with the melatonin rhythm. Relative to the aligned cycle, leptin fell about 17 percent, postprandial glucose rose about 6 percent despite insulin rising about 22 percent, mean arterial pressure rose about 3 mm Hg, and sleep efficiency fell about 20 percent; three of eight participants showed a postprandial glucose response in the post-meal prediabetic range (Scheer, Hilton, Mantzoros, and Shea, 2009). Morris and colleagues later separated circadian phase from behavioural cycle and again found adverse glucose and cardiovascular profiles under misalignment (Morris et al., 2015; Morris, Purvis, Hu, and Scheer, 2016). Buxton and colleagues combined sleep restriction with circadian disruption in a multi-week laboratory study and reported reduced insulin sensitivity and a fall in resting metabolic rate (Buxton et al., 2012).

Those protocols control meals and, often, activity. They are the cleanest human evidence that timing itself can move metabolic markers. They last days to weeks. They are not a thirty-year night-shift career, and they are not a licence to call every late dinner “circadian damage.”

Mouse genetics enlarge the mechanism and the overclaim. Clock mutant mice develop obesity and metabolic syndrome features (Turek et al., 2005). Bmal1 and liver- or pancreas-specific deletions disturb glucose handling (Rudic et al., 2004; Lamia, Storch, and Weitz, 2008; Marcheva et al., 2010). Adipocyte Clock deletion alters energy regulation (Paschos et al., 2012). Animal papers stay animal papers. Bass and Takahashi reviewed the integration of circadian and metabolic cycles without converting those knockouts into a human diet (Bass and Takahashi, 2010).

Sleep restriction, without an explicit circadian shift, is a parallel metabolic literature. Spiegel, Leproult, and Van Cauter showed that recurrent partial sleep restriction in young men reduced glucose tolerance and acute insulin response (Spiegel, Leproult, and Van Cauter, 1999). Spiegel and colleagues later reported lower leptin and higher ghrelin after sleep restriction (Spiegel et al., 2004). Tasali, Leproult, Ehrmann, and Van Cauter selectively suppressed slow-wave sleep and reduced insulin sensitivity without changing sleep duration (Tasali, Leproult, Ehrmann, and Van Cauter, 2008). A late sleeper who is also a short sleeper occupies both literatures at once. The article will not pretend they are one literature.

17 Glucose

Insulin sensitivity is worse in the biological evening than in the biological morning in controlled protocols (reviewed in Qian and Scheer, 2016). That circadian modulation is why an identical meal is not an identical glucose excursion at 08:00 and 20:00. It is also why early time-restricted feeding can move glycaemic markers when calories are held (Sutton et al., 2018; Jamshed et al., 2019). Wehrens and colleagues delayed meals by five hours and, in a subsequent constant routine, found a delayed glucose rhythm and a shifted adipose PER2 rhythm while melatonin and cortisol — the SCN markers — stayed put (Wehrens et al., 2017). Meals timed the periphery. They did not time the pacemaker.

The glucose claim that survives is narrow and strong: phase of eating and phase of the clock can change glycaemia without a change in meal composition. The claim that does not survive is that a branded eating window is diabetes care, or that clock time replaces carbohydrate quality, total energy, or medication. Liu and colleagues’ calorie-matched trial of time-restricted eating added to restriction did not improve weight loss over restriction alone (Liu et al., 2022). That trial is a weight trial. It still bounds the slogan that timing is an independent metabolic drug.

18 Obesity

Obesity associations with short sleep, late eating, and social jet lag are plentiful and residual-confounded. Roenneberg’s social-jet-lag BMI association is questionnaire epidemiology (Roenneberg et al., 2012). Garaulet and colleagues found that among people in a weight-loss programme, those who ate lunch after 15:00 lost less weight over twenty weeks despite similar reported energy intake, composition, estimated expenditure, and sleep duration (Garaulet et al., 2013). Jakubowicz and colleagues assigned an isocaloric larger breakfast versus larger dinner and reported greater weight and triglyceride change with the morning-loaded pattern (Jakubowicz, Barnea, Wainstein, and Froy, 2013). Those are timing signals. They are not a demonstration that calories dropped out of the equation.

Late 16:8 windows have failed to beat control for weight in important trials (Lowe et al., 2020). Early windows sometimes add a little when calories are not tightly constrained, consistent with a spontaneous intake reduction (Jamshed et al., 2022). The obesity remainder after energy is accounted for is small, conditional, and early-window-shaped — the same remainder SBL-41/SP-FASTING already recorded. This title will not reopen that ledger as if clocks had repealed it.

19 Cardiovascular disease

Vyas and colleagues’ systematic review associated shift work with increased risk of myocardial infarction and other vascular events (Vyas et al., 2012). Vetter and colleagues, in the Nurses’ Health Studies, associated longer duration of rotating night-shift work with a higher risk of coronary heart disease (Vetter et al., 2016). Knutsson’s earlier occupational reviews had already placed cardiovascular disease on the shift-work list (Knutsson, 2003). Kecklund and Axelsson reviewed the broader health consequences of shift work and insufficient sleep together (Kecklund and Axelsson, 2016).

The association is the finding. The mechanism is a list: sleep curtailment, nocturnal eating, light-at-night, smoking and BMI differences, socioeconomic selection into night work, and, possibly, circadian misalignment of the kind Scheer produced in the laboratory. Morris, Purvis, Hu, and Scheer showed that circadian misalignment can raise blood pressure and inflammatory markers in a short laboratory protocol (Morris, Purvis, Hu, and Scheer, 2016). That is a plausible intermediate. It is not a thirty-year causal proof. §36 returns the verdict.

20 Cancer epidemiology

IARC convened on shift work in 2007 and classified “shiftwork that involves circadian disruption” as probably carcinogenic to humans (Group 2A) (Straif et al., 2007). A later article (Volume 124) kept night-shift work in Group 2A (Ward et al., 2019). Group 2A is not Group 1. It means limited human evidence plus sufficient animal evidence, or a similarly cautious combination. The human core is breast cancer in night-working women, with mixed cohort results. Schernhammer and colleagues reported positive associations in nurses (Schernhammer et al., 2001; Schernhammer, Kroenke, Laden, and Hankinson, 2006). Travis and colleagues, in the Million Women Study and other large UK cohorts, did not find an association of night-shift work with breast cancer (Travis et al., 2016). Ijaz and colleagues’ systematic review judged the breast-cancer evidence inconsistent (Ijaz et al., 2013). Hansen has reviewed the same collision of positive and null studies (Hansen, 2017).

Mouse clock-gene papers — Per2 mutant tumour predisposition (Fu, Pelicano, Liu, Huang, and Lee, 2003); circadian disruption accelerating tumours (Filipski et al., 2004; Papagiannakopoulos et al., 2016) — explain why IARC could find animal sufficiency. They do not decide a human cohort. Sulli, Manoogian, Taub, and Panda have reviewed circadian–cancer biology as a mechanistic field (Sulli, Manoogian, Taub, and Panda, 2018). Mechanistic richness is not epidemiological closure.

The honest sentence is: night work remains a Group 2A exposure; the human breast-cancer signal is not settled; other sites are thinner; clock-gene mice are not night nurses.

21 Cognition

Alertness and some cognitive operations vary with circadian phase and with time awake. Van Dongen, Maislin, Mullington, and Dinges showed that chronic partial sleep restriction accumulates cognitive deficits that people poorly perceive (Van Dongen, Maislin, Mullington, and Dinges, 2003). Circadian modulation of cognition is visible in forced-desynchrony and constant-routine protocols (Wright, Hull, and Czeisler, 2002). Walker’s reviews of sleep and memory concern sleep stages more than clock genes (Walker, 2009). Livingston and colleagues included sleep as a modifiable dementia-risk domain at population scale (Livingston et al., 2020). That is a sleep-and-dementia claim, not a demonstration that a 05:30 wake time prevents Alzheimer’s disease.

Laboratory circadian misalignment can impair mood and some performance metrics in the same protocols that worsen glucose (Scheer et al., 2009; Chellappa, Morris, and Scheer, 2018). Those are days-long studies in healthy adults. They are not cognitive aging trials.

22 Mood

Seasonal affective disorder entered the literature as a syndrome that bright light could treat (Rosenthal et al., 1984). Golden and colleagues’ meta-analysis found a significant effect of light therapy for seasonal affective disorder and a smaller, still positive, effect for non-seasonal depression (Golden et al., 2005). Terman and Terman reviewed timing and dose-response for light as a psychiatric intervention (Terman and Terman, 2005). Wirz-Justice and colleagues placed light, sleep deprivation, and melatonin among chronotherapeutics for depression (Wirz-Justice and Benedetti, 2020). McClung reviewed circadian genes and mood-circuit biology (McClung, 2013).

The SAD light-therapy signal is the strongest interventional mood claim in this article. It is a timed light claim, not a general “circadian optimization” claim. Non-seasonal depression evidence is thinner, heterogeneous, and not a substitute for ordinary psychiatric care. This document recommends no treatment.

23 Immunity

Sleep and circadian phase both time immune function. Lange, Dimitrov, and Born reviewed sleep’s regulation of immune signalling (Lange, Dimitrov, and Born, 2010). Besedovsky, Lange, and Haack updated the sleep–immune relationship (Besedovsky, Lange, and Haack, 2019). Scheiermann, Kunisaki, and Frenette described circadian control of leukocyte trafficking (Scheiermann, Kunisaki, and Frenette, 2013). Labrecque and Cermakian reviewed clocks inside immune cells (Labrecque and Cermakian, 2015).

Human clinical endpoints are fewer: vaccine-response studies after sleep restriction, infection-risk associations with short sleep, and time-of-day variation in some inflammatory markers. Those are real and small relative to the molecular literature. A melatonin or “circadian immune support” product claim does not follow.

24 Aging

Human circadian amplitude often declines with age; phase often advances; sleep fragments (Duffy, Zitting, and Chinoy, 2015). Hood and Amir reviewed clocks and aging as a bidirectional problem: aging changes clocks, and clock disruption can resemble aging in animals (Hood and Amir, 2017). Kondratova and Kondratov reviewed Bmal1 and other clock-gene aging phenotypes in mice (Kondratova and Kondratov, 2012). A premature-aging mouse knockout is not a human healthspan series. No circadian intervention has an accepted human effect on disability-free life or HALE — the vocabulary SBL-41/SP-HEALTHSPAN already forced those words to keep.

25 Health-outcome matrix

26 Light therapy

Bright light is a timed stimulus with a PRC, a psychiatric history, and an occupational one. The load-bearing clinical use is seasonal affective disorder (Rosenthal et al., 1984; Golden et al., 2005; Terman and Terman, 2005). Morning light is the usual SAD protocol because it is intended to advance a delayed winter phase. That rationale is chronobiological. The outcome is a depression scale, not a melatonin curve, in most trials.

Light is also used, with weaker and more condition-specific evidence, for some circadian rhythm sleep–wake disorders and for adaptation toward a desired phase (Morgenthaler et al., 2007; Auger et al., 2015). Intensity, spectrum, duration, and — above all — internal time decide the sign of the shift. A lamp used at the wrong phase delays the person who bought it to advance. This document does not prescribe illuminance, duration, or clock time.

27 Darkness and light avoidance

The other half of the light PRC is the evening. If evening light delays, then dimming evening melanopic light is a delay-avoidance manoeuvre. Gooley 2011 and Chang 2015 are the mechanistic human papers. Wright’s camping studies show that a week of natural light–dark can compress late chronotypes toward an earlier melatonin phase (Wright et al., 2013; Stothard et al., 2017). Those are small field studies with a large light contrast. They do not show that blackout curtains add healthy years.

Amber lenses and “blue-blocker” glasses are discussed in §38. The scientific object is evening melanopic lux, not a particular plastic colour.

28 Melatonin as a chronobiotic

Used as a hypnotic, melatonin is a weak, short-latency drug in meta-analysis (Ferracioli-Oda, Qawasmi, and Bloch, 2013). Used as a chronobiotic, it is a PRC-guided phase signal (Lewy et al., 1992; Burgess et al., 2010). Jet-lag reviews have long treated timed melatonin as one of the few interventions with a randomized evidence base (Herxheimer and Petrie, 2002; Arendt, 2009). Circadian rhythm sleep–wake disorder guidelines discuss melatonin for some delayed and free-running indications (Morgenthaler et al., 2007; Auger et al., 2015). Insomnia guidelines are cooler (Sateia et al., 2017).

The distinction matters because consumer melatonin is sold as sleep, not as a timed zeitgeber. A tablet taken at a random evening hour is not “supporting the clock.” It may be taking a small soporific at an unknown phase. No dose or schedule is given here.

29 Meal timing

Meal timing is a peripheral-clock intervention that sometimes has a glycaemic residue after energy is matched and often has a weight residue only when energy is not. Sutton, Beyl, Early, Cefalu, Ravussin, and Peterson’s five-week early time-restricted feeding crossover in men with prediabetes used a six-hour eating window with dinner before 15:00 versus a twelve-hour window, on weight-maintaining controlled diets, and reported improved insulin sensitivity, β-cell responsiveness, blood pressure, and oxidative-stress markers without weight loss (Sutton et al., 2018). Jamshed and colleagues’ four-day crossover moved 24-hour glucose and a set of transcripts (Jamshed et al., 2019). Xie and colleagues’ five-week parallel trial found early TRF superior to mid-day TRF for insulin sensitivity (Xie et al., 2022). Hutchison and colleagues found that TRF improved a test-meal glucose response whether early or delayed, with only the early window lowering mean fasting continuous-glucose values (Hutchison et al., 2019).

The calorie-matched and calorie-unmatched weight literature is owned in detail by SBL-41/SP-FASTING. The sentences required here are only these: an early eating day can be a circadian intervention; a late 16:8 is usually not; Liu 2022 found no 12-month weight advantage of adding a window to calorie restriction; Lowe’s TREAT trial of a noon-to-20:00 window was null between groups (Liu et al., 2022; Lowe et al., 2020). St-Onge and colleagues’ American Heart Association scientific statement treated meal timing as a cardiovascular research topic, not as a completed therapy (St-Onge et al., 2017).

Mouse time-restricted feeding can prevent obesity on a high-fat diet without reducing calories (Hatori et al., 2012). Circadian-aligned calorie restriction can extend mouse lifespan beyond misaligned restriction (Acosta-Rodríguez et al., 2022). Those are important animal papers. They are not human lifespan results.

30 Exercise timing

Exercise can shift circadian phase in humans, but the effects are smaller and less reliably signed than light’s (Atkinson, Edwards, Reilly, and Waterhouse, 2007; Youngstedt, Elliott, and Kripke, 2019; Thomas et al., 2020). Morning versus evening training studies often confound chronotype, meal timing, and sleep. Performance itself has a circadian profile — better in the late biological afternoon for many strength and power tasks — which is a reason to time sport, not a reason to claim that evening training “wrecks the clock.” Exercise remains a first-line metabolic and cardiovascular intervention in the exercise articles of this series. It is a second-line zeitgeber here.

31 Shift-work strategies

Occupational night work is a collision of sleep restriction, nocturnal eating, light-at-night, and social isolation. Strategies studied in laboratory simulations and small field trials include scheduled light and darkness to shift the pacemaker toward the night, timed naps, and, in some protocols, timed melatonin (Eastman and Martin, 1999; Smith, Fogg, and Eastman, 2009; Boivin, Boudreau, and Kosmadopoulos, 2022). Permanent nights with a committed nocturnal life can, in principle, entrain. Rotating shifts repeatedly ask the PRC to do work it cannot finish in two days.

None of those strategies has been shown, in a large randomized occupational trial, to remove the cardiovascular or cancer associations in §19–§20. They are harm-reduction hypotheses. They are not a medical clearance for nights. This document does not prescribe a roster.

32 Jet lag

Jet lag is a transient misalignment after a time-zone shift. Eastward travel generally requires an advance and is harder for a pacemaker whose free-running period is slightly longer than twenty-four hours (Czeisler et al., 1999; Waterhouse, Reilly, Atkinson, and Edwards, 2007). Westward travel requires a delay and is, on average, easier. Timed light, dark, and melatonin have randomized and guideline-level support as phase-shifting tools (Herxheimer and Petrie, 2002; Sack et al., 2007; Arendt, 2009; Eastman and Burgess, 2009). The PRC, not the airline meal, decides the sign.

Jet lag is the cleanest real-world analogue of laboratory misalignment: the clock is intact, the social day jumped. It is still temporary. Using jet-lag tactics as a daily lifestyle is a confusion of timescales.

33 Meal-timing matrix

34 Shift-work matrix

35 Circadian optimization

“Circadian optimization” treats amplitude, phase, internal alignment, sleep duration, and a morning routine as one knob. They are not one knob. A person can be well slept and late. A person can be early and internally misaligned if meals sit in the biological night. A person can have a high-amplitude melatonin rhythm and coronary disease. Laboratory misalignment shows that some knobs move some markers in days (Scheer et al., 2009). It does not show that a consumer stack of light, supplements, and 05:00 alarms produces those laboratory conditions, or that those laboratory conditions extend healthspan.

The phrase also smuggles a maximum. Biology here is about adequacy and timing, not about optimizing a score. There is no accepted human circadian biomarker whose improvement has been shown to add disability-free years.

36 Causality from shift-work epidemiology

Night work is associated with metabolic, cardiovascular, and — at IARC 2A — carcinogenic outcomes. That sentence stands. The next sentence often written is that circadian disruption causes those outcomes. The designs do not isolate that cause.

Healthy-worker and survivor effects run in both directions: people who cannot tolerate nights leave; people who remain may be different at baseline. Socioeconomic position, smoking, BMI, diet, alcohol, and sunlight exposure travel with the roster. Sleep duration and sleep timing travel together. Light-at-night is both a circadian stimulus and a marker of urban poverty and of the job itself. Few cohorts measure melatonin phase. IARC’s animal sufficiency can be true while the human occupational contrast remains a bundle (Straif et al., 2007; Ward et al., 2019). Travis 2016 is the required null-leaning breast-cancer paper for any reader who has only been shown Schernhammer (Travis et al., 2016; Schernhammer et al., 2001).

Laboratory misalignment supplies biological plausibility, not occupational attribution. Plausibility is not surplus causation.

37 Exact universal wake and sleep timing

There is no clock time that is healthy for every human. Chronotype varies; it late-shifts in adolescence and earlier-shifts with age (Roenneberg et al., 2004; Fischer et al., 2017). The free-running period is near twenty-four hours and not identical across people (Czeisler et al., 1999; Duffy and Czeisler, 2002). Latitude, season, and evening light history move phase. A 05:00 wake that places one person in the morning light-advance zone places another, who fell asleep at 03:00, in sleep restriction.

School and work start times are social choices with distributional consequences, especially for late adolescents. That is a policy literature adjacent to this title. It is not a reason to invert the error and declare that late types must be left entirely unscheduled. It is a reason to stop selling a single wake time as biology.

38 Blue-light marketing

Melanopsin is short-wavelength-sensitive (Brainard et al., 2001; Thapan et al., 2001; Berson et al., 2002). Marketing translated that action spectrum into a morality play about “blue light” as a toxin and about amber products as its antidote. The laboratory facts that actually move melatonin are intensity, duration, and timing, with spectrum as a modifier (Zeitzer et al., 2000; Lucas et al., 2014; Gooley et al., 2011; Chang et al., 2015). A dim blue indicator is not equivalent to an overhead LED room. A bright warm room can suppress melatonin. Photopic lux is the wrong meter; melanopic EDI is a better one (Brown et al., 2022). Neither meter is a disease endpoint.

Souman and colleagues reviewed acute alerting effects of short-wavelength light and found the cognitive story smaller and messier than the advertisements (Souman, Tinga, te Pas, van Ee, and Vlaskamp, 2018). Wood, Rea, Plitnick, and Figueiro measured melatonin effects of self-luminous displays (Wood, Rea, Plitnick, and Figueiro, 2013). Those papers support evening-light hygiene as a real variable. They do not support a specialty-glass subscription as circadian medicine.

39 Chronotype moralizing

Morningness is rewarded by office hours. Eveningness is punished by them. The language that follows — lazy, undisciplined, “no excuse” — is a status language, not a physiological one. Extreme delayed sleep–wake phase disorder is a clinical entity with guideline-level discussion (Auger et al., 2015). Ordinary late chronotype is a population trait (Roenneberg et al., 2007; Fischer et al., 2017). Conflating the two is how a teenager’s biology becomes a character indictment.

The moralizing also runs the other way: early types congratulated as aligned, optimized, and virtuous. An early type who sleeps five hours is not aligned. A late type who sleeps eight hours on a delayed schedule may be, internally, the healthier of the two. Duration, regularity, and phase angle to that person’s clock are the measurements. Wall-clock virtue is not.

40 Food timing without calories or diet quality

The surviving timing residue is early placement of the eating day and the laboratory fact that meals can shift peripheral clocks (Wehrens et al., 2017; Sutton et al., 2018; Xie et al., 2022). The non-surviving claim is that when one eats is a substitute for what and how much. Liu 2022 is the required calorie-matched long trial (Liu et al., 2022). Lowe 2020 is the required late-window null (Lowe et al., 2020). Garaulet 2013 and Jakubowicz 2013 keep timing visible after crude energy matching in shorter or observational settings (Garaulet et al., 2013; Jakubowicz et al., 2013). They do not authorize a nutrition-free circadian diet.

Mouse papers in which time-restricted feeding prevents obesity at equal calories (Hatori et al., 2012) are the usual citation for the slogan. They are mice, on a high-fat diet, with nocturnal feeding aligned to a nocturnal species. Humans are diurnal. The translation is a hypothesis, not a completed one.

41 Unresolved

The food-entrainable oscillator’s anatomy is unresolved. The fraction of shift-work disease that would remain if sleep, smoking, and BMI were perfectly matched is unresolved. The human breast-cancer night-work signal is unresolved as between Schernhammer and Travis. The long-term effect of early time-restricted eating on hard outcomes is unresolved. The effect of evening-light reduction on cardiovascular events is untested. Whether increasing circadian amplitude in older adults changes function is unresolved. Whether melanopic lighting standards will move population health is unresolved. Whether any circadian intervention changes healthspan, as SBL-41/SP-HEALTHSPAN defined it, is not even asked in an adequate trial.

42 How to read the remainder

Keep the word for the oscillator. Let light time the SCN. Let meals time the liver. Let sleep duration stay a separate sentence from sleep phase. Let shift-work epidemiology stay an association with a Group 2A cancer classification and a cardiovascular signal, not a completed causal story. Let SAD light therapy stay a timed psychiatric intervention. Let meal timing stay a possible glycaemic modifier that does not repeal energy balance. Refuse a universal wake time, a blue-light morality, a chronotype hierarchy, and an optimization brand.

That is the whole remaining claim.

References