# Your body clock: how circadian rhythms control sleep Source: https://sleepfacts.uk/how-sleep-works/body-clock/ Author: Tess Marlow ยท Updated 2026-11-04 **Short answer:** A master clock in the brain runs on a cycle of just over 24 hours and is reset every day, mainly by light reaching your eyes. It times when you feel sleepy, when melatonin is released and when your body temperature dips. Bright light in the morning helps you sleep at night; bright light late in the evening pushes your clock later. ## Where is the body clock? In a tiny region of the brain called the suprachiasmatic nucleus, just above where the nerves from the eyes cross. It works as a master clock. Almost every organ, from the liver to the gut, has its own clock too, and the master clock keeps them in step. Left alone, without daylight or clocks, the human body clock runs slightly long. In a careful lab study of young and older adults living in controlled lighting, it averaged 24.18 hours. So every day it needs a small nudge to stay matched to the 24-hour day, and that nudge comes mainly from light. ## How does light set the clock? Special cells in the retina, separate from the ones you see with, respond to light, especially blue-rich light, and send signals straight to the master clock. - **Light in the morning** moves the clock earlier, so you feel sleepy earlier that evening. - **Light in the late evening and night** moves it later and holds back melatonin. That is why daylight soon after waking is one of the most useful things you can do for your sleep, and why bright screens and rooms late at night can work against you. In one lab study, people who read on a light-emitting tablet before bed for five evenings released less melatonin, had a later body clock and took longer to fall asleep than when they read a printed book. See [screens and blue light](/sleep-better/screens-and-blue-light/). Outdoor light is far brighter than indoor light, even on a grey British day, so a walk in the morning does more than sitting by a lamp. ## What does the clock control? - **Melatonin.** Levels rise a couple of hours before your usual bedtime, peak in the middle of the night and fall towards morning. - **Body temperature.** Core temperature drops in the evening and reaches its low point a couple of hours before you normally wake. That fall helps you fall asleep, which is why a [warm bath](/sleep-better/warm-bath/), which makes you shed heat afterwards, can help. - **Alertness.** You are most alert in the late morning and early evening, with a small dip in the early afternoon. - **Hormones, digestion and blood pressure** all follow daily rhythms too. ## What happens when your clock is out of step? If you sleep at a time your clock thinks is daytime, sleep is shorter and lighter. That happens with: - **[Jet lag](/sleep-problems/jet-lag/),** when you fly across time zones faster than the clock can adjust, usually about an hour a day. - **[Shift work](/sleep-problems/shift-work/),** especially nights. Most night workers' clocks never fully adjust, because they see daylight on the way home and on days off. - **Social jet lag,** the gap between when your clock wants you to sleep and when work or school makes you get up. It is largest in teenagers and [evening types](/how-sleep-works/larks-and-owls/), who often catch up at weekends. ## How can you keep your clock steady? - Get up at the same time every day, including weekends. - Get outside in daylight in the morning. - Dim the lights in the last hour or two before bed, and keep screens dim or at a distance. - Keep the bedroom dark. Blackout curtains or an eye mask help in summer, when it is light before 5am in much of the UK. - Eat meals at regular times. ## Frequently asked questions ### How do I reset my body clock? Get bright light, ideally daylight outdoors, soon after you wake, and keep evenings dim. Get up at the same time every day, including weekends, even after a bad night. Most people's clocks shift by about an hour a day with consistent light and timing. ### Does melatonin reset the body clock? It can nudge it. Taken in the early evening, small doses of melatonin move the clock earlier, which is why it helps jet lag. In the UK melatonin is prescription only. See our melatonin guide. ### Why do I wake up at the same time every night? Often because you come up into light sleep at the end of a cycle and notice something, such as noise, light, needing the toilet or worry. If you then lie awake for long periods on most nights, that may be insomnia, which responds well to CBT. ## References 1. [Czeisler CA et al. Stability, precision, and near-24-hour period of the human circadian pacemaker. Science, 1999.](https://pubmed.ncbi.nlm.nih.gov/10381883/) 2. [Saper CB, Scammell TE, Lu J. Hypothalamic regulation of sleep and circadian rhythms. Nature, 2005.](https://pubmed.ncbi.nlm.nih.gov/16251950/) 3. [Chang AM et al. Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. PNAS, 2015.](https://pubmed.ncbi.nlm.nih.gov/25535358/) 4. [Roenneberg T et al. Epidemiology of the human circadian clock. Sleep Medicine Reviews, 2007.](https://pubmed.ncbi.nlm.nih.gov/17936039/) 5. [Kecklund G, Axelsson J. Health consequences of shift work and insufficient sleep. BMJ, 2016.](https://pubmed.ncbi.nlm.nih.gov/27803010/)