Circadian Rhythms — The Body's Internal Clock
Without any clock or sunlight, your body knows approximately what time it is — your temperature, cortisol, and sleep drive oscillate with near-perfect 24-hour precision. This is the circadian rhythm, encoded in feedback loops of gene expression that have been conserved for 3 billion years.
1. Discovery and Overview
The first controlled demonstration of a biological clock was by Jean-Jacques d'Ortous de Mairan in 1729: he showed that mimosa plants kept in constant darkness continued to open and close their leaves on a roughly 24-hour cycle — an endogenous (internal) oscillator, not just a response to the sun.
Today we know circadian clocks are present in nearly every cell type and are conserved across cyanobacteria, fungi, plants, insects, and mammals. The 2017 Nobel Prize in Medicine went to Hall, Rosbash, and Young for uncovering the molecular mechanism in fruit flies.
Core temperature rises. Cortisol peak. Alertness begins.
Cognitive performance peak. Reaction times fastest.
Muscle strength & cardiovascular peak. Core temperature max.
Melatonin onset. Temperature falls. Sleep drive building.
2. The SCN Master Pacemaker
In mammals, the master pacemaker is the suprachiasmatic nucleus (SCN) — a pair of tiny nuclei in the hypothalamus containing about 20 000 neurons each. If the SCN is lesioned, all circadian rhythms collapse. If SCN tissue is transplanted into an arrhythmic animal, rhythms return with the period of the donor.
The SCN sits directly above the optic chiasm and receives direct light input from intrinsically photosensitive retinal ganglion cells (ipRGCs) containing melanopsin. This is the primary entrainment pathway: photons → SCN → body clock reset.
3. Molecular Feedback Loop
The clock mechanism is a transcriptional-translational feedback loop (TTFL):
-
Morning: Transcription factor heterodimer
CLOCK:BMAL1 binds E-box sequences and drives
period (
PER1/2/3) and cryptochrome (CRY1/2) gene expression. - Afternoon: PER and CRY proteins accumulate in the cytoplasm. CKIε/δ phosphorylates PER, targeting it for degradation (controls delay).
- Evening: Sufficient PER:CRY complex forms, translocates to the nucleus, and inhibits CLOCK:BMAL1 — switching off its own transcription.
- Night: PER and CRY proteins are degraded. BMAL1 transcription rises (via ROR activators outcompeting REV-ERB repressors).
- Next morning: CLOCK:BMAL1 activity is restored. The cycle (~24 h) repeats.
dP/dt = v₃·M − v₄·P (protein dynamics)
dR/dt = v₅·P − v₆·R (repressor dynamics)
Period ≈ 24 h depends on: Hill coefficient n ≥ 9
delay from transcription, translation, phosphorylation & degradation.
The key insight: the loop needs delay and non-linearity (high Hill coefficient n) to sustain oscillations. Mutations that speed up PER degradation (short-period familial advanced sleep phase syndrome) or slow it (delayed sleep phase disorder) shift the clock by hours.
4. Light Entrainment
The free-running period of the human clock is not exactly 24 hours — it averages ~24.2 hours. Daily light exposure entrains (resets) the clock to the solar day.
The phase response curve (PRC) describes how a light pulse shifts the clock:
- Light in the early subjective night (before core temperature minimum ≈ 4 AM): phase delay — clock shifts later.
- Light in the late subjective night (after 4 AM): phase advance — clock shifts earlier.
- Light at midday: almost no shift (dead zone).
This is why bright light in the morning helps early risers; blue-light blocking glasses in the evening prevent phase delays from screens.
5. Melatonin and the Dark Signal
The pineal gland secretes melatonin exclusively during the dark phase (typically 9 PM–7 AM). Melatonin is not a "sleep hormone" — it is a darkness hormone that signals night duration to body tissues.
Melatonin synthesis pathway: tryptophan → serotonin → N-acetylserotonin → melatonin (by AANAT + HIOMT enzymes). The SCN inhibits AANAT expression during the day through noradrenergic pathways; removal of this inhibition at night allows melatonin production.
Exogenous low-dose melatonin (0.5–3 mg) can shift the clock: taken in the afternoon it phase advances; taken in the morning it phase delays — the opposite of light. This makes it clinically useful for jet lag and blind individuals who lack light input.
6. Mathematical Models
Limit-cycle oscillators are used to model circadian clocks. The van der Pol oscillator and Goodwin oscillator are classical models; more detailed Leloup-Goldbecker 16-variable model captures mammalian TTFL with high accuracy.
θᵢ = phase of neuron i
ωᵢ = intrinsic angular frequency (~2π/24.2 h)
K = coupling strength between neurons
Z(θ) = phase response curve
I(t) = light input signal
The Kuramoto model (used for Kuramoto oscillator simulation) captures how 20 000 SCN neurons synchronise via neuropeptide VIP coupling despite having slightly different intrinsic periods (σ ≈ 0.5 h). Without coupling, the population desynchronises within days.
7. Jet Lag and Shift Work
Jet lag occurs when the internal clock is misaligned with local time. Flying eastward requires phase advance (harder — your clock must compress a day). Flying westward requires phase delay (easier — extending the day).
Re-entrainment rate is ~1 hour per day for westward travel, ~0.5–0.75 hours per day for eastward travel. A 12-hour shift (New York → Tokyo eastward) takes about 2 weeks to fully recover.
Light exposure protocols for jet lag: on the day of eastward travel, seek bright light in the morning and avoid it in the evening for 2–3 days before departure (gradually advancing the clock). Low-dose melatonin at destination bedtime accelerates re-entrainment.
8. Applications
- Chronotherapy: Timing drug administration to the circadian phase when it is most effective or least toxic. Blood pressure medications work best in the morning (morning hypertension peak). Cancer chemotherapy timed to cell cycle phase and circadian metabolic peaks can improve efficacy by 30–50%.
- Chronotype assessment: The Munich Chronotype Questionnaire (MCTQ) quantifies individual chronotype (morning lark vs. night owl). Chronotype has a strong genetic basis: over 350 loci associated with sleep timing have been identified in GWAS studies.
- Artificial light design: Tunable LED systems (‟human-centric lighting") adjust colour temperature through the day — blue-enriched morning light (5000 K) for alertness, amber evening light (2700 K) to avoid melatonin suppression.
- Simulation: Mathematical models of the circadian clock are used to design optimal work/rest schedules for long-duration spaceflight, where light cycles must be artificially managed.