How to Reset Your Body Clock: Circadian Rhythm and Melatonin

Last updated: September 2026

A body clock that has drifted out of step with your day is fixable, and usually faster than people expect. The tools are not complicated: light at the right times, darkness at the right times, and a wake time that does not move. This guide explains what the circadian rhythm and melatonin actually are, and the steps that reset them.

What is the circadian rhythm?

The circadian rhythm is the internal clock, running on a cycle of roughly 24 hours, that tells your body when to be sleepy, alert, hungry and warm. It is kept by a cluster of nerve cells called the suprachiasmatic nucleus in the hypothalamus, which sits just above the point where the optic nerves cross, so its main time signal is the light reaching your eyes.

That wiring is the whole trick to resetting it. Morning light tells the clock the day has started and pulls everything earlier; bright light in the late evening tells it the day is still going and pushes everything later. The clock also drives a nightly drop in core body temperature, which is one reason a cool, dark bedroom makes sleep come more easily.

What is melatonin and what does it do?

Melatonin is the hormone the pineal gland releases in response to darkness, signalling to the rest of the body that night has arrived. Levels start rising a couple of hours before your usual bedtime, peak in the early hours and are switched off by morning light. It does not knock you out like a sedative; it opens the gate that lets sleep happen.

Two practical points follow. First, evening screens and bright overhead lighting delay the rise, which is why the last two hours before bed matter more than any supplement. Second, melatonin production declines with age, which is one reason sleep often gets lighter in later life. In the UK melatonin is a prescription-only medicine, usually prescribed short-term for adults over 55, so it is not something to self-dose from the internet; the NHS advises speaking to a GP if sleep problems persist beyond a few weeks.

How do you reset your body clock?

Anchor the wake time first: get up at the same time every day, weekends included, because the clock sets itself by when the day starts, not when it ends. Everything else supports that anchor.

Light does most of the heavy lifting. Get outside, or at least to a bright window, within an hour of waking; in the evening, dim the main lights and keep screens low and warm for the last two hours. The clearest demonstration of how strongly light rules the clock is a University of Colorado study by Wright and colleagues, published in Current Biology in 2013, which found that one week of camping without artificial light re-synchronised participants' body clocks to the sun, with melatonin onset shifting around two hours earlier. You do not need a tent; you need bright mornings and dim evenings.

Move gradually. Shift bedtime and wake time by 15 to 30 minutes a day towards the schedule you want rather than making one big jump, which the clock resists. Keep naps to 20 minutes and before 3 pm so they do not drain the pressure to sleep at night, and hold the routine steady for one to two weeks; consistency is what convinces the clock the new schedule is real. Our guides on falling asleep fast and staying asleep all night cover the two ends of the night, and if you are catching up after a run of short nights, the sleep debt guide explains why earlier bedtimes beat weekend lie-ins. Shift workers have a harder version of the same problem, covered in our night shift sleep guide.

What foods help melatonin production?

Tart cherries are the best-evidenced dietary source of melatonin, and small trials have linked tart cherry juice with modest improvements in sleep time. Beyond that, foods rich in tryptophan (turkey, milk, eggs, oats) and magnesium (nuts, seeds, leafy greens) supply the raw materials the body uses to make melatonin and serotonin.

Keep the claims in proportion: no food will overpower a bright screen at midnight. Food choices are a supporting act; light and timing are the leads. The same goes for caffeine, which blocks the sleep-pressure chemical adenosine for six hours or more, so an afternoon cut-off protects the schedule you are trying to build.

How do clock changes and light mornings affect sleep?

The twice-yearly clock change is a one-hour jet lag for the whole country, and the same gradual-shift rule handles it: move bedtime and wake time by 15 to 30 minutes a day in the few days before the change rather than absorbing the hour in one night, and take bright morning light on the new schedule to lock it in.

Light summer mornings and evenings pull the clock the other way, waking you at 5 am or keeping you up past midnight. Blackout curtains or a contoured eye mask restore the darkness signal in the morning, and dim, warm lighting after sunset preserves the evening melatonin rise even when it is still bright outside. The room itself is part of the signal: dark, quiet and around 16 to 18 degrees is the environment the clock expects at night.

Does your bed affect your body clock?

Indirectly but meaningfully, because the nightly drop in core temperature is one of the circadian signals for sleep, and a mattress that traps heat works against it. A surface that lets warmth escape helps your body make the temperature drop the clock is trying to schedule.

The REM-Fit 3000 Supreme is built for exactly that: open-cell memory foam that lets air pass through the foam so heat and moisture move away from the body, over 3000 tall premium pocket springs whose mostly-air core lets the mattress breathe. At a medium 6 out of 10 it suits side sleepers, and like every model in the range of REM-Fit hybrid mattresses it holds a steady, cool surface through the early hours when your temperature is at its lowest.

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Body clock and melatonin FAQs

Last updated: September 2026

How long does it take to reset your body clock?

Around one to two weeks for most schedule drifts, moving 15 to 30 minutes a day with a fixed wake time and bright morning light. As a rule of thumb the clock shifts about an hour a day at best, so a big change such as night-shift work or long-haul travel takes proportionally longer.

Can you buy melatonin in the UK?

Not over the counter. Melatonin is a prescription-only medicine in the UK, usually prescribed short-term for adults over 55 with insomnia. Speak to a GP rather than buying it online, and treat light exposure and a fixed wake time as the first-line fix.

Does pulling an all-nighter reset your sleep schedule?

No. Staying up all night adds sleep debt on top of a shifted clock, and the crash that follows usually lands at the wrong time and makes the drift worse. Shift gradually with a fixed wake time instead; it takes a few days longer and actually holds.

Why do light mornings wake me up so early?

Because morning light shuts down melatonin and signals the start of the day, even at 5 am through closed eyelids and thin curtains. Blackout curtains or an eye mask restore the darkness signal so your clock runs to your schedule rather than the sun's.

What is the best light for sleep in the evening?

Dim and warm. Low, warm-toned lamps in the last two hours before bed let melatonin rise on time, while bright overhead or blue-toned light delays it. Keep screens low, use night mode, and make the bedroom itself as dark as you can.

Sources: Wright et al., Entrainment of the human circadian clock to the natural light-dark cycle, Current Biology, 2013; NHS, Melatonin; Sleep Foundation, Circadian Rhythm.