Body clock

Melatonergic circadian entrainment — the alignment of the internal clock with the day–night cycle.

Sleep hormone increases it.

Melatonin/SCN study 2001, Am J Physiol Cell Physiol 280:C110 — melatonin phase-advances the suprachiasmatic circadian clock via MT2 receptor signaling. doi 10.1152/ajpcell.2001.280.1.C110

Lowers cortisol increases it.

Cortisol is a secondary entraining signal that coordinates local clocks throughout the body to the master clock: basal glucocorticoids peak at the habitual waking time, but only when the SCN is intact, and glucocorticoids induce Per1 through a glucocorticoid-response element in the Per1 promoter (PMC7744424). Note: this is a phase-setting relationship, not a more-is-better one — a cortisol rhythm entrains the clock; a flattened or chronically raised one does not. A direction caveat: this link is drawn from the HPA axis to the circadian clock, but the dominant direction in this literature is the reverse. HPA hormones exhibit ~24-hour rhythms that are primarily driven by the central circadian clock in the suprachiasmatic nucleus; cortisol peaks shortly after waking and declines through the day because the SCN sets it. The forward arrow is still defensible — HPA dysregulation flattens and phase-shifts the diurnal profile, and disrupted circadian rhythm is listed among the drivers of HPA dysfunction — so the true relationship is reciprocal. Only one direction is drawn here. The HPA axis is itself an AUTOREGULATORY negative feedback loop: cortisol negatively regulates CRH and ACTH. REFUTED ON A SECOND READING. This link was shown by linked steps until a source was opened that does not support it: “The hormones of the HPA axis exhibit regular approximately 24 h periodic rhythms that are primarily driven by the central circadian clock located in the hypothalamic suprachiasmatic nuclei (SCN).” — Modeling the Influence of Chronic Sleep Restriction on Cortisol Circadian Rhythms, with Implications for Metabolic Disorders. Journal name, volume and pages NOT READ (title READ via pubannotation.org full text). doi:10.3390/metabo11080483 | PMID 34436424 |. doi 10.3390/metabo11080483 · PMID 34436424 · PMC7744424

Longevity gene increases it.

SIRT1 is a circadian deacetylase acting on core clock components, and hypothalamic functions including circadian rhythmicity are regulated by SIRT1 (PMC4040287, reviewing Asher 2008, Nakahata 2008 and Belden & Dunlap, Cell 2008;134:212). This is the metabolic arm of the clock: the same NAD-dependent enzyme that reads energy status also sets time. The literature contradicts itself on this edge's sign and both sides ARE recorded here. Activator: SIRT1 in the brain governs central circadian control by activating the transcription of the two major circadian regulators, BMAL1 and clock, forming an amplifying loop with PGC-1-alpha and Nampt (Cell 2013). Repressor: SIRT1 acts as a negative regulator by deacetylating BMAL1 and histones at circadian gene promoters to promote repression by the inhibitory limb of the oscillator, and separately deacetylates PER2 to facilitate its ubiquitination and degradation. The aging evidence is the strongest part and is not disputed: in aged wild-type mice SCN SIRT1 falls, BMAL1 and PER2 fall with it, and the animals show a longer intrinsic period, a more disrupted activity pattern, and an inability to adapt to changes in light entrainment. Aging lowers NAD+, which lowers SIRT1, which reduces circadian amplitude and desynchronises peripheral clocks. PMC4040287

Fasting response increases it.

The decisive experiment removes the master clock entirely: in SCN-lesioned mice, in-vivo recording of Bmal1-driven luciferase in hepatocytes showed that after only 6 days of daytime restricted feeding the liver clock had undergone a steady and complete phase shift to match the food intake rhythm — while sham-operated mice had only partially shifted in the same time. The intact SCN was actively fighting the food signal. Speed and specificity: clock-gene rhythms in liver and cerebral cortex entrain within 2 days, before food-anticipatory activity can develop, and the liver is the most rapidly and sensitively entrained tissue. And here is the finding that decides how this edge is drawn: natural food intake patterns have little synchronising effect on peripheral circadian clocks. Mimicking a mouse’s own ad-libitum pattern and shifting it by 12 hours advanced peripheral clocks by only 0-3 hours. Scheduled meals are not a zeitgeber; a long fast is. The authors name the reason: TRF is not a normal eating pattern, and it imposes non-physiologic long fasts. Glucocorticoid dependence ties it to an edge already drawn: the asymmetry in resetting speed does not occur in adrenalectomised OR glucocorticoid-insensitive mice. PMID 33158435 · PMC7182033

Training load increases it.

A proper human phase-response curve exists, which is rare for any non-photic CUE: 51 older and 48 young adults each performed 60 minutes of moderate treadmill exercise at 65-75 percent of heart rate reserve, at one of eight times of day OR night, for three consecutive days, with urinary 6-sulphatoxymelatonin tracked. Large phase delays from 7PM to 10PM, large phase advances at 7AM and from 1PM to 4PM, minimal shifts around 4pm and 2am. NO significant AGE OR sex differences. And the magnitude is the headline: the amplitudes of the phase-response curves ARE comparable to expectations for bright light of equal duration. Exercise is not a weak accessory cue — on these data it is in the same class as the strongest circadian intervention there is. Independently replicated with a different marker: in 52 sedentary adults measured by DIM light melatonin onset, morning exercise advanced phase by 0.62 hours versus -0.02 for evening (P = 0.01). And CHRONOTYPE overrides the rule: for later chronotypes, both morning and evening exercise induced phase advances. Advice built on the average curve will be wrong for night owls. An unresolved species dissociation: mouse studies strongly argue that exercise affects the peripheral clock but not the central clock — treadmill running advanced the skeletal muscle clock by up to 8 hours with NO detected shift in the SCN. Human work shows exercise moving the melatonin rhythm, taken to reflect the central clock. Both cannot be simply true. doi 10.1172/jci.insight.134270 · PMC6462487

Sleep pressure increases it.

The human trial is unusually well built: double-blind, placebo-controlled, within-subject, running about 49 days. A caffeine dose equivalent to a double espresso taken 3 hours before habitual bedtime induced a roughly 40-minute phase delay of the circadian melatonin rhythm. And the comparison is the line worth remembering: that delay was nearly half the magnitude of the phase delay produced by three hours of evening bright light at about 3000 LUX. One evening coffee does half of what a light box does — in the wrong direction. The receptor is named and isolated in vitro: chronic caffeine lengthens the circadian period of molecular oscillations in human U2OS cells carrying bmal1:luciferase, primarily through an adenosine receptor / cyclic-AMP-dependent mechanism, with period lengthening reproduced by the adenosine receptor antagonist CGS-15943 alone. Replicated in mice with a cleaner tool: a selective adenosine A1 receptor antagonist reproduced caffeine’s enhancement of photic phase-delays. Sleep regulation is classically taught as two independent processes — homeostatic and circadian. This edge shows they share a node. The molecule that tracks how long you have been awake also sets the clock that decides when you should be. central versus peripheral again: acute caffeine shifts clock genes in the liver and kidney but not in the SCN. doi 10.1126/scitranslmed.aac5125

Sleep increases it.

It was the control condition in a study designed to test something else, which is often where the cleanest result hides. Investigators acclimatised mice to scheduled feeding over three months in constant darkness, removing the light-dark cycle entirely, then asked what predicted peripheral clock phase. The answer was not the food: peripheral clock phases were better predicted by the rest-activity cycle than by the food schedule, contrary to expectation based on time-restricted-feeding studies. Now the Sleep → Circadian Rhythm link closes it. The clock sets the sleep-wake cycle and the sleep-wake cycle feeds back on the clock. an association within a controlled experiment, not a manipulation of sleep followed by measurement of the clock. The rest-activity cycle also carries body temperature, corticosterone and locomotor signals that were not separated — and the HPA Axis / Cortisol → Circadian Rhythm link and the Exercise → Circadian Rhythm link are drawn independently. PMID 33158435 · PMC7646075

Builds mitochondria increases it.

The source paper closes with the claim in its own words: we have thus identified PGC-1-alpha as a key component of the circadian oscillator that integrates the mammalian clock and energy metabolism. Not an input to the clock — part of it. The driver is itself rhythmic: PGC-1-alpha is rhythmically expressed in the liver and skeletal muscle of mice, with protein peaking in the early dark phase. The mechanism is named at the level of the transcription factor: PGC-1-alpha stimulates the expression of clock genes, notably Bmal1 and Rev-erb-alpha, through coactivation of the ROR family of orphan nuclear receptors. And loss of function was demonstrated at three independent levels: whole-animal — mice lacking PGC-1-alpha show abnormal diurnal rhythms of activity, body temperature and metabolic rate; tissue-specific — knockout in liver and skeletal muscle attenuates rhythmic expression of clock and PER1; and cell-autonomous — PGC-1-alpha-deficient fibroblasts and liver-specific knockdown indicate it is required for cell-autonomous clock function. The rhythm fails in a dish, not just in an animal. It sits alongside the SIRT1 → Circadian Rhythm link, already drawn: SIRT1 acts on the same machinery from the other side, binding and deacetylating BMAL1 and PER2. Two metabolic sensors, two routes, one oscillator. That should be searched on its own evidence rather than assumed. doi 10.1038/nature05767 · PMID 17476214

Vision increases it.

10.1038/nature06829 — This link rests on the reviews of this outcome as a whole. No effect size specific to this arm has been reported, so it is shown at reduced weight. (from the reviews of this outcome). doi 10.1038/nature06829

Thyroid increases it.

10.3389/fphys.2021.706433 — This link rests on the reviews of this outcome as a whole. No effect size specific to this arm has been reported, so it is shown at reduced weight. (from the reviews of this outcome). doi 10.3389/fphys.2021.706433

It increases Lowers cortisol.

Reverse ARROWThe atlas drew only the HPA Axis / Cortisol → Circadian Rhythm link, the weaker leg. The lesion evidence is definitive: loss of the circadian rhythm of corticosterone after suprachiasmatic nucleus lesions provided some of the first evidence that the SCN is the brain's master clock; ablating SPZ-Vgat neurons eliminates the circadian rhythm of corticosterone secretion; and the SCN is required for the daily rhythm of plasma glucocorticoids (PMC11261983; PMC3718825). Photic activation of the SCN raises plasma glucocorticoids. Route: SCN -> subparaventricular zone -> dorsomedial hypothalamus -> PVH CRH neurons, plus rhythmic SCN vasopressin, plus a splanchnic nerve line direct to the adrenal. Control is bimodal, not one-way: the adrenal has its own intrinsic clock driving cyclic steroidogenesis. Daytime restricted feeding uncouples the two and produces two separate glucocorticoid peaks — one tied to adrenal content, one requiring an intact SCN. And photic SCN activation raised glucocorticoids without a functional adrenal molecular clock, so part of the pathway is ACTH-independent and neural (Sci Rep 2017;7:46404 / PMC5388859). Species: rat and mouse. PMC11261983

It increases Mood.

The causal work is as strong as anything on this pathway. Clinical: treatments directly targeting the circadian system — acute sleep deprivation, bright light therapy and sleep phase advance — can be successful in treating depression. You can treat this illness by moving the clock. Molecular and causal, two independent ways of breaking it: targeted deletion of BMAL1 in CaMK2a excitatory neurons of the medial prefrontal cortex abolished the behavioral and molecular response to sleep deprivation; and pharmacological activation of the clock repressor REV-ERB suppressed the antidepressant effects of sleep deprivation. Knock out the clock gene or drug the clock repressor — either way the antidepressant stops working. That is the clock being necessary for a treatment to act. A genuine paradox, recorded rather than smoothed: sleep deprivation and ketamine ARE both rapid-acting antidepressants, yet they exert opposing effects on mPFC circadian gene expression — SD enhances the negative clock-loop genes Per and Cry, mirroring stress, while ketamine downregulates them. Any account claiming a single clock-gene direction for depression is ignoring one of the two. PMC12916475

It reduces Calm under stress.

The causal paper was designed specifically to rule out the usual CONFOUNDS: previous work used light manipulation (light affects mood non-circadianly), global clock-gene mutants (clock genes have pleiotropic clock-independent functions) or brain lesions (which destroy cells and connections). This study used AAV-delivered short hairpin RNA to knock down Bmal1 only in the suprachiasmatic nucleus. Result: disrupting SCN circadian rhythms is sufficient to cause helplessness, behavioral despair and anxiety-like behavior in mice. One clock manipulation, two outcomes, only one of them moved. Converging: optogenetic SCN stimulation at unpredictable times produced dampened, fragmented and unstable rhythms; Period 1 and Period 2 in the nucleus accumbens regulate anxiety-related behavior; clock-gene polymorphisms associate with anxiety disorders in humans. The authors own caveat: there is conflicting evidence as to whether the SCN directly modulates mood and anxiety — the route may run through other oscillators or through loss of internal synchrony. PMID 27113500 · PMC3807562

It reduces Attention regulation.

Distinct from the Sleep → ADHD link and the Melatonin → ADHD link because the measurements are of the clock itself. Delayed sleep phase disorder is present in 73-78 percent of children and adults with ADHD, and delayed sleep-wake timing in up to 78 percent. The molecular clock is measurably altered: attenuated peripheral clock-gene rhythms of BMAL1 and PER2, alongside blunted and delayed cortisol rhythms and reduced pineal volume. Genetics: clock-gene polymorphisms including per and clock are linked to ADHD. Same direction caveat as the Sleep → ADHD link: ADHD is either the cause OR the consequence of these disturbances, or they share aetiology. The arrow is drawn this way because the intervention evidence — bright light and melatonin advancing DLMO with correlated symptom improvement — runs this way. PMC12728042

It increases Sleep.

Cell-type-specific and causal: targeted deletion of the core clock gene BMAL1 in CaMK2a-expressing excitatory neurons of the medial prefrontal cortex disrupted sleep-wake architecture and elevated slow-wave activity. Not a correlation between rhythm and rest — remove one clock gene from one cell type and the structure of sleep changes. And at the master pacemaker: SCN Bmal1 knockdown shortened the period and dampened the amplitude of activity rhythms, while chronic optogenetic SCN stimulation at unpredictable times produced dampened, fragmented and unstable rhythms. It is not all of it: the note on sleep and mood records that one night of deliberate deprivation is antidepressant while chronic loss is not, and that homeostatic arm is not a clock phenomenon. PMID 27113500 · PMC12916475

It reduces Supports weight management.

The title is a causal claim the paper earns: genetic and environmental circadian disruption induce weight gain through changes in the gut microbiome. Two independent models agree: mice with BMAL1 deficiency in the suprachiasmatic nucleus and wild-type mice under simulated shift work both gained weight, both showing desynchronisation between peripheral gastrointestinal clocks and reduced microbial rhythmicity. And the transfer experiment makes it mechanistic: germ-free colonization with shift-work-associated microbiota mechanistically links body weight gain to the microbial changes. The recipients never did shift work. The recipients also showed altered peripheral clock gene expression — the transplanted microbiota reset the host clock, which is the loop closing. Bidirectional and the authors say so: conversely, body weight changes induce microbiota dysbiosis. doi 10.1016/j.molmet.2022.101628 · PMC8291023

It increases Insulin sensitivity.

Controlled laboratory studies have shown that short-term circadian misalignment and sleep restriction independently impair physiological processes, including insulin sensitivity, energy expenditure, immune function, blood pressure and cardiac modulation by the AUTONOMOUS nervous system. The laboratory design separated them. The mechanism is meal timing and it is measured: feeding rodents only during the rest phase can phase shift peripheral oscillators in the liver, pancreas, kidney and heart while it has NO effect on the phase of the SCN. Meal timing moves the organs and not the master clock — which is precisely how internal desynchrony is manufactured, and why shift workers eating at 3am are not simply tired people. doi 10.1111/jsr.12770 · PMC3704149

It increases Sympathetic drive.

Controlled laboratory studies have shown that short-term circadian misalignment and sleep restriction independently impair physiological processes, including insulin sensitivity, energy expenditure, immune function, blood pressure and cardiac modulation by the AUTONOMOUS nervous system. The laboratory design separated them. The anatomy is specified: multisynaptic projections from the SCN to the liver, pancreas, adrenal cortex and adipocytes have been discovered, providing neuroanatomical pathways by which the SCN can influence metabolic-related factors, and the SCN entrains the adrenal clock through hormonal cues and sympathetic innervation via the splanchnic nerve. This is an anatomical connection, not a correlation. doi 10.1111/jsr.12770 · PMC3704149

It increases Immune support.

Controlled laboratory studies have shown that short-term circadian misalignment and sleep restriction independently impair physiological processes, including insulin sensitivity, energy expenditure, immune function, blood pressure and cardiac modulation by the AUTONOMOUS nervous system. The laboratory design separated them. The rhythm is in the immune system itself: circadian regulation of immune responses is evident in the rhythmic expression of cytokines and the fluctuation in the number and activity of immune cells. And it has an actionable consequence the reviewers name: chronomodulated vaccine schedules represent a promising strategy to improve vaccine efficacy — time of day as a dosing variable. doi 10.1101/gad.352180.124 · PMC12529610

It reduces Heat instead of storage.

Energy expenditure appears directly in the controlled-laboratory list: controlled laboratory studies have shown that short-term circadian misalignment and sleep restriction independently impair physiological processes, including insulin sensitivity, energy expenditure, immune function, blood pressure and cardiac modulation by the AUTONOMOUS nervous system. The laboratory design separated them. The source says energy expenditure without specifying the thermogenic mechanism, so this edge carries the outcome and not the route (doi 10.1111/jsr.12770; PMC12164040). REFUTED ON A SECOND READING. This link was shown by linked steps until a source was opened that does not support it: “Disruption of the brown adipocyte clock impairs thermogenesis. || mildly reduced both BAT thermogenesis and whole-body energy expenditure” — Brown adipocyte-specific knockout of Bmal1 causes mild but significant thermogenesis impairment in mice. Molecular Metabolism, 2021, 49, 101202. doi 10.1111/jsr.12770 · PMID 33676029 · PMC12164040

It increases Memory & focus.

Different studies indicated that chronic jet LAG influences the elevated level of cortisol, poor cognitive functions, low working memory, learning problems, and higher rates of breast and prostate cancer. Corroborated at review level: disturbances in circadian rhythms ARE associated with mood and cognitive disorders. Weaker than the four edges drawn alongside it: those rest on controlled laboratory separation of misalignment from sleep restriction; this one rests on chronic-jet-lag observational work in which the two are confounded. Drawn, but at a lower grade, and the confound is the reason. doi 10.37349/emed.2022.00093 · PMC12529610

It reduces Neurovascular calm.

Named across six migraine pathophysiology reviews. Migraine pathogenesis is widely accepted to be attributed to neuroinflammation and sensitization of the trigeminovascular system. PMID 36361791 · PMC10933035

It reduces Fasting insulin.

insulin, fasting insulin, type 2 diabetes and blood lipids share one literature: heart, kidney, liver, and metabolic diseases share interlinked pathological mechanisms, including inflammation, oxidative stress, insulin resistance, and endothelial health issues. These four overlap by nature: fasting insulin is the level, insulin is the signaling, type 2 diabetes is the diagnosis and blood lipids are the substrate. The same driver legitimately reaches all four, and a reader should not mistake four arrows for four independent findings. doi 10.1038/s41581-026-01066-0 · PMC11725408

It increases Stress cascade trigger.

10.1038/s41467-021-25959-9 — Drawn from the reviews of this outcome. doi 10.1038/s41467-021-25959-9

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