How to fix your circadian rhythm

Your body clock runs slightly slow. Left in constant dim light with no cues, the average human circadian rhythm settles at 24.18 hours — about eleven minutes longer than a day. That gap is small, and it is why light matters so much: every morning, your clock has to be nudged back into line with the world. Miss that nudge often enough and the drift compounds. That is when sleep starts arriving later, mornings get harder, and the whole thing feels stuck.

The good news is that the fix is mostly free. This guide covers what your circadian rhythm actually is, why light is the lever that matters more than any other, how much morning light you realistically need, what the evidence does and doesn’t show about screens at night, and where caffeine, exercise, meals and melatonin fit in. It also covers when a stubborn body clock stops being a lifestyle problem and becomes something to see your GP about.

What your circadian rhythm actually is

Your master clock is a pair of tiny structures in the hypothalamus called the suprachiasmatic nuclei — together about a quarter of a cubic millimetre. They keep near-24-hour time on their own, and they set the timing of melatonin release, core body temperature, alertness and a long list of other things.

They also take instruction from the eye. A subset of retinal cells contains a pigment called melanopsin, most sensitive to blue light around 480 nanometres, and these cells run a direct line to the clock. They aren’t for seeing. They’re for telling the clock what time of day it is.

In 1999, a Harvard team measured the intrinsic period of that clock under controlled conditions and found it averaged 24.18 hours. Earlier studies had put it nearer 25 hours, but those had let participants choose their own lighting, which skewed the result. Women’s clocks run marginally faster than men’s — 24.09 hours against 24.19.

Morning light: get outside, and don’t overthink the dose

The reason light dominates everything else is dose. Under office lighting you’re getting around 500 lux, often less. Full daylight is about 25,000 lux and bright sunlight can reach 100,000. Outdoors, your clock is reading a signal fifty to two hundred times stronger.

Researchers in Colorado measured this in the field. Over a week of winter camping, participants averaged 10,297 lux during waking hours against 752 lux in their normal electrically lit lives — more than thirteen times as much light. After that week, melatonin onset had moved 2.6 hours earlier. Summer camping does it too, and faster than you would expect: a single weekend moved melatonin onset 1.4 hours earlier, which the researchers put at 69% of the shift they had recorded after a full week of it.

Timing decides direction. Light in the hours after your core body temperature minimum — roughly the last stretch before you normally wake — pulls the clock earlier. Light before that point, meaning late evening and night, pushes it later.

The dose-response is forgiving. In laboratory work, a 6.7-hour bright light pulse produced a maximum shift of just over five hours; cut that to a single hour at around 8,000 lux and you still get 2.20 hours — 40% of the effect from 15% of the exposure. Most of the benefit is front-loaded. So getting outside soon after waking, for something in the region of 30 minutes, is the version worth doing. A UK clinical review of delayed sleep-wake phase disorder recommends either a 10,000 lux lamp for about half an hour on waking, or simply getting into daylight as fast as you can after waking, for 30 minutes. An overcast British morning comfortably beats any indoor lighting you own.

Sit by a window if you have to, but glass and distance cost you a lot of lux. Walking to the station, eating breakfast in the garden, taking a coffee outside — all count. So does the dog.

Evening light: dim the room, not just the screen

This is where popular advice has run ahead of the evidence, in both directions.

The effect is real, and it starts at lower light levels than most people assume. In one study of 55 young adults, the light level that suppressed melatonin by half was 24.6 lux — dimmer than a typical living room. Five hours of evening light at 10, 30 and 50 lux delayed melatonin onset by 22, 77 and 109 minutes respectively. The much-quoted eReader study found that four hours of evening reading on a lit screen, five nights running, suppressed melatonin by 55%, delayed melatonin onset by more than an hour and a half, added about ten minutes to how long it took to fall asleep, and cut REM sleep by around twelve minutes. Worth noting how extreme that protocol is: four solid hours, every evening, against a comparison group reading paper in near-darkness. It is not the same as twenty minutes on your phone.

Two other findings deserve more attention than they get. First, individual sensitivity varies enormously — in that same study the dose needed for 50% melatonin suppression ranged from 6 lux in the most sensitive person to 350 lux in the least. The same lamp is not the same signal to two different people. Second, the fixes people reach for are the weaker ones. Warm-tinting a screen without turning the brightness down did not significantly change melatonin suppression in testing. And a 2025 meta-analysis of randomised trials of blue-blocking glasses found a non-significant 4.86-minute reduction in time to fall asleep, with a confidence interval running from a 20-minute improvement to a 10-minute worsening.

None of which makes amber glasses pointless. An international expert consensus recommends keeping light below 10 melanopic lux in the three hours before bed, and anything that helps you hit that is fair game. Blockers like BON CHARGE Blue-Light Glasses are an easy add-on, and if wearing them is what makes you take evening light seriously, they earn their place. They are the accessory. The intervention is turning the big lights off.

Fix your wake time first

NHS insomnia guidance is specific about which end to anchor: “wake up and get out of bed at the same time every day.” Not bedtime. There is a mechanistic reason — a fixed wake time fixes the timing of your morning light exposure, which is the thing doing the resetting. Bedtime is largely a consequence of how tired you are and where your clock currently sits.

Regularity turns out to matter in its own right. In a UK Biobank analysis of 60,977 people followed for just over six years, those in the most regular quintile of sleep timing had a 30% lower rate of death from any cause than the least regular, and sleep regularity predicted the outcome better than sleep duration did. That is an observational finding, so it shows association rather than cause. It is a striking association all the same.

Caffeine, exercise, meals and a warm bath

Caffeine does more than keep you awake — it moves the clock. A double-espresso dose taken three hours before bed produced a 40-minute circadian phase delay, roughly half the delay caused by three hours of bright evening light. Separately, 400 mg of caffeine taken six hours before bed still significantly disrupted sleep compared with placebo. Caffeine’s half-life is 3 to 7 hours in adults, so the afternoon flat white is still in your system at midnight. NHS advice is to avoid caffeine for at least six hours before bed; the Sleep Charity says eight.

Exercise has its own phase response curve. Sixty minutes of moderate exercise advanced the clock when done around 7am or between 1pm and 4pm, and delayed it between 7pm and 10pm, with an effect size the researchers described as comparable to bright light of the same duration. A morning run is doing two jobs at once.

Meal timing is the one that gets over-claimed. When researchers delayed participants’ meals by five hours, the blood glucose rhythm shifted with them — but melatonin didn’t move at all. Meal timing shifts your metabolic clocks, not your brain clock. Eating late may still be worth avoiding for other reasons; it just won’t reset your body clock.

A warm bath is the cheapest trick here. A review of 17 studies found bathing at 40–42.5 °C, one to two hours before bed, for as little as ten minutes, was associated with falling asleep faster and better sleep efficiency. The mechanism is the drop in core temperature afterwards, which is one of the signals sleep follows.

Jet lag, shift work, and melatonin

Left to itself, your clock shifts by up to about 1.5 hours a day when moving later and about 1 hour a day when moving earlier, which is why flying east is worse than flying west. Well-timed light exposure improves that to roughly 2 hours and 1.5 hours respectively. The NHS notes jet lag usually settles within a few days, and advises getting outside during the day so natural light can do the adjusting.

On melatonin, UK readers should know the rules here are different. Melatonin is prescription-only in the UK, licensed mainly for short-term sleep problems in adults aged 55 and over — typically 2 mg prolonged-release, taken one to two hours before bed, for up to 13 weeks. Use for circadian rhythm disorders such as delayed sleep-wake phase is a specialist sleep clinic decision. The NHS is blunt about jet lag: melatonin tablets “are not recommended on the NHS for jet lag because there’s not enough evidence to show they work.”

As a general sleep aid, the effect is real but modest. Pooling 19 studies covering 1,683 people, melatonin cut the time to fall asleep by 7.06 minutes and added 8.25 minutes of total sleep. As a clock-shifter rather than a sedative it is more interesting, and there the timing matters more than the dose — the largest advances came from taking a low dose in the afternoon, several hours before melatonin naturally rises. That is a conversation to have with a clinician, not something to improvise.

When to see a GP

Some late clocks aren’t a habit. Delayed sleep-wake phase disorder affects around 1% of adults across population surveys, and considerably more young people — 3.3% of Norwegian adolescents and 4.6% of a Swedish sample aged 16 to 26. If you consistently cannot fall asleep before the early hours and cannot wake at a normal time, and it is damaging your work or education, that is worth raising rather than white-knuckling.

NHS guidance is to see a GP if changing your sleep habits hasn’t helped, or if you have had trouble sleeping for months. A GP will look for an underlying cause and may offer cognitive behavioural therapy; sleeping pills are now rarely prescribed for insomnia.

Some symptoms need a GP regardless of what your body clock is doing. If your breathing stops and starts while you sleep, if you make gasping, snorting or choking noises, or if you always feel very tired during the day, that points towards obstructive sleep apnoea, which can be serious if it isn’t diagnosed and treated.

Frequently asked questions

How long does it take to fix a circadian rhythm?

It depends how far out you are. Unaided, the clock moves up to about 1.5 hours later or 1 hour earlier per day; with well-timed light you can push that to roughly 2 hours and 1.5 hours. A one-hour correction is a couple of days. A four-hour correction is a week or more of consistency.

What is the fastest way to reset your body clock?

Daylight in the morning, dim light in the evening, and the same wake time every day. A week of winter camping shifted melatonin onset 2.6 hours earlier, and a single summer weekend got 69% of the way to the equivalent week-long shift. You don’t need a tent, but you do need to get outside.

Does blue light really stop you sleeping?

Evening light suppresses melatonin and delays the clock, and it starts at lower levels than most people think — around 25 lux for a 50% effect on average. But sensitivity varies from 6 to 350 lux between individuals, and the effects in screen studies come from hours of exposure, not minutes.

Do blue light glasses work?

The randomised trial evidence is weak. A 2025 meta-analysis found a non-significant 4.86-minute reduction in time to fall asleep. They may help you keep total evening light down, which is the thing that actually matters.

Is it better to have a fixed bedtime or a fixed wake time?

A fixed wake time. That is what NHS insomnia guidance specifies, and it is what fixes the timing of your morning light exposure.

Can I buy melatonin in the UK?

No. Melatonin is prescription-only in the UK. It is licensed mainly for short-term sleep problems in adults aged 55 and over, and use for circadian rhythm disorders is a specialist decision.

How late can I drink coffee?

NHS advice is at least six hours before bed; the Sleep Charity says eight. Caffeine’s half-life is 3 to 7 hours, and 400 mg taken six hours before bed still measurably disrupted sleep in testing.

Does eating late affect your body clock?

It shifts your metabolic rhythms — a five-hour meal delay moved blood glucose timing substantially — but it did not shift melatonin, so it doesn’t move the master clock.

Does exercise help reset your circadian rhythm?

Yes. Sixty minutes of moderate exercise advanced the clock around 7am and between 1pm and 4pm, with an effect comparable to bright light of the same duration. Evening exercise between 7pm and 10pm pushed it later.

When should I see a doctor about my body clock?

If changing your habits hasn’t helped, or you have had trouble sleeping for months. Also see a GP straight away if your breathing stops and starts in your sleep, you gasp or choke at night, or you are always very tired during the day.

The bottom line

Fixing a circadian rhythm is less about discipline than about signal strength. Your clock runs eleven minutes slow and resets itself on light, so the whole job is making the morning signal loud and the evening signal quiet.

Get outside in the morning, ideally within an hour of waking and for something like half an hour. Turn the overhead lights down in the evening rather than relying on a screen setting. Keep your wake time fixed, even at weekends. Move your last coffee earlier than feels necessary. And if you want to exercise for the clock as well as the body, do it in the morning or early afternoon.

If your sleep timing has been badly out for months, or you have symptoms that point to something other than a drifting clock, this is general information rather than medical advice — see your GP.