The Molecular Circadian Clock
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Direct answer
A liver cell cultured in a dish keeps ticking for days with a period near, but not exactly, 24 hours — which is why the body needs a master clock to synchronise its trillions of peripheral oscillators. That master is the suprachiasmatic nucleus, a pair of hypothalamic nuclei of roughly 20,000 neurons entrained by light through the retinohypothalamic tract from melanopsin-containing retinal ganglion cells (peak sensitivity near 480 nm). Inside every clock cell runs a transcription-translation feedback loop: CLOCK and BMAL1 proteins drive transcription of Period and Cryptochrome genes, whose protein products accumulate, enter the nucleus and repress their own transcription until they degrade — one full cycle taking about a day, tuned by casein kinase 1 phosphorylation. Output rhythms with exam weight: cortisol peaking 06:00-08:00 with its nadir near midnight, melatonin rising two to three hours before sleep and peaking 02:00-05:00, and core temperature minimum around 04:00-05:00 — the timing shift work scrambles.
What you must remember
- Core loop: CLOCK-BMAL1 heterodimers bind E-box DNA elements driving Per and Cry transcription; PER and CRY proteins feed back to shut off their own genes, then degrade over roughly 24 hours — a self-sustaining, temperature-compensated oscillator.
- Period tuning by kinases: casein kinase 1 delta and epsilon phosphorylate PER proteins to mark them for degradation; the tau mutant hamster's 20-hour day and human familial advanced sleep phase syndrome both trace to this step.
- Entrainment hierarchy: light synchronises the suprachiasmatic nucleus within days; peripheral clocks (liver, muscle, adipose) follow the master clock but are also set by meal timing, glucocorticoids and temperature.
- Melatonin pathway: suprachiasmatic nucleus to paraventricular nucleus to intermediolateral column to superior cervical ganglion to pineal; N-acetyltransferase is the rate-limiting enzyme, and light striking the retina suppresses it immediately.
- Hormonal timing anchors: melatonin onset near 21:00 (levels fall steeply with age); thyroid-stimulating hormone peaks in the evening; leptin rises overnight.
- Clock disorder anchors: jet lag resynchronising at only about 1-1.5 hours per day eastward (harder than westward); shift work classified by the International Agency for Research on Cancer as probably carcinogenic (Group 2A) for circadian disruption; delayed sleep phase syndrome of adolescence.
- Chronotherapy hooks: once-daily corticosteroids dosed in the morning to respect the cortisol peak, evening theophylline for nocturnal asthma, and blood pressure medicines timed to the morning surge — the standard exam examples.
Working through a night-shift roster
A junior resident rotating to a week of night duty finds herself alert at 03:00 and unable to sleep at 09:00 after handover. Sequence the clock biology: her suprachiasmatic nucleus is still reset by morning light on the commute home while her sleep drive is weakest at the temperature minimum near 05:00 — she sleeps at a phase that actively suppresses sleep architecture. Strategies that follow the physiology: wear dark glasses on the morning drive to limit phase-shifting light, anchor sleep in a dark cool room or split it with a late-afternoon nap, use bright light early in the shift, and accept that full adaptation needs long consistent blocks — which rotating rosters never allow, the reason fixed shifts are recommended. The same logic applied to pharmacology: melatonin given in the early evening advances the clock (useful eastbound), while morning melatonin delays it — the direction, often asked, depends on timing, not dose.
Where students slip
The suprachiasmatic nucleus is memorised as the site and nothing more; examiners want the loop — name CLOCK, BMAL1, PER and CRY, say which represses which, and cite the 2017 Nobel Prize to Hall, Rosbash and Young. Second, students declare melatonin "the sleep hormone" and stop; its onset (dim light melatonin onset, about two to three hours before habitual sleep) is the measurable phase marker, and its suppression by light is the mechanism by which evening screens delay sleep. Third, peripheral clocks are forgotten: the liver clock follows food, so irregular night eating can desynchronise liver metabolism from the master clock — the proposed link between shift work and metabolic disease, quotable as mechanism with appropriate hedging. Finally, do not claim clocks are light-dependent; they are self-sustaining in constant darkness — light only entrains.
Frequently asked questions
Which proteins form the core circadian feedback loop?
CLOCK-BMAL1 heterodimers drive Period and Cryptochrome transcription, and accumulating PER and CRY proteins repress CLOCK-BMAL1 until they degrade, completing an approximately 24-hour cycle.
How does light entrain the master clock?
Melanopsin-containing retinal ganglion cells with peak sensitivity near 480 nm project via the retinohypothalamic tract to the suprachiasmatic nucleus, resetting its phase.
Why is melatonin synthesis called rate-limited by N-acetyltransferase?
Pineal serotonin is converted to N-acetylserotonin by N-acetyltransferase, the enzyme whose sympathetic (noradrenaline-driven) induction at night and immediate suppression by light set melatonin timing.
What causes familial advanced sleep phase syndrome?
Period gene mutations accelerating PER degradation through casein kinase 1 phosphorylation, shortening the clock's period and driving very early sleep and wake times.
Which drug timings exploit circadian rhythms?
Morning corticosteroid dosing to mimic the cortisol peak and spare the axis, evening theophylline for nocturnal asthma, and antihypertensive timing against the morning blood pressure surge.