Melatonin for Circadian Rhythm

Melatonin is best understood not as a sleeping pill but as a clock signal. Your pineal gland releases it as evening light falls, and the brain reads that rise as a message about what time it is biologically. Taken as a supplement at the right hour, it shifts that clock; taken at the wrong hour, it can shift it the wrong way. This is why the evidence for melatonin on circadian outcomes such as delayed sleep phase, shift work and non-24-hour rhythm disorders is stronger than the evidence for it as a sedative. Phase response curve studies show clear, dose-independent shifts of about an hour when melatonin is taken in the hours before habitual bedtime. The practical consequence: timing is the active ingredient. A 0.5 mg dose taken five hours before bed does more for a misaligned clock than 10 mg taken at lights-out.

Melatonin also does not replace light. Morning bright light and evening light avoidance are the stronger levers for realigning a clock, and melatonin works best layered on top of them. Used together, the combination is the standard approach to delayed sleep phase: dim light and low-dose melatonin in the evening, bright light on waking, and a fixed wake time defended every day including weekends.

The suprachiasmatic nucleus in the hypothalamus keeps roughly 24-hour time and is entrained mainly by light reaching intrinsically photosensitive retinal ganglion cells. It drives pineal melatonin secretion, and melatonin in turn feeds back onto MT1 and MT2 receptors in the same nucleus. That feedback loop is what supplementation exploits. Exogenous melatonin in the evening mimics an earlier dusk and advances the clock; melatonin in the morning mimics a later dusk and delays it. MT1 signalling reduces neuronal firing in the suprachiasmatic nucleus, producing the mild sleepiness people notice, while MT2 signalling carries most of the phase-shifting effect. Melatonin also lowers core body temperature slightly and promotes distal vasodilation, both physiological precursors of sleep onset.

For circadian goals, the useful doses are far smaller than what most shops sell. Physiological doses of 0.3 to 0.5 mg produce phase shifts equal to 3 mg or more, while larger doses spill melatonin across a wider window and can blur the timing signal. Anchor timing to dim light melatonin onset, which occurs roughly two hours before your natural sleep time. To advance a late clock, take melatonin four to six hours before habitual sleep onset, then move bedtime and wake time earlier by 15 to 30 minutes every few days as the clock follows.

The clearest evidence comes from phase response curve work: three days of daily melatonin at 0.5 mg and 3.0 mg produced comparable advances of roughly an hour, with the size and direction of the shift determined almost entirely by administration time relative to internal clock phase. Systematic reviews of primary sleep disorders reinforce the pattern. Melatonin reliably improves sleep timing in circadian rhythm disorders, with much smaller effects on sleep-onset latency and total sleep time in ordinary insomnia. The jet lag literature is the practical proof of concept, showing faster resynchronisation across multiple time zones.

Short-term use is well tolerated. Reported effects are headache, vivid dreams, next-morning grogginess at higher doses, and mild dizziness. Grogginess is largely a dose and timing problem and usually disappears at 0.5 mg. Longer-term safety data are thinner, particularly in children and adolescents where there is theoretical concern about effects on pubertal timing, though trials in neurodevelopmental conditions have not demonstrated harm. Product quality is a real issue: analyses of over-the-counter melatonin have found actual content varying widely from the label, and some products containing serotonin.

Melatonin is cleared largely by CYP1A2, so anything that inhibits that enzyme raises exposure substantially. Fluvoxamine is the standout example, increasing melatonin levels many-fold. Smoking has the opposite effect, inducing CYP1A2 and lowering exposure.