Circadian rhythm

Your body runs on a 24-hour schedule whether you notice it or not. That schedule is your circadian rhythm. The term comes from Latin: circa, meaning “around”, and diem, meaning “day”. It is the internal process that regulates the sleep–wake cycle, hormone release, body temperature, digestion and even mood, repeating roughly every 24 hours.

How it works: the master clock

The control centre lives in your brain’s hypothalamus, in a group of about 20,000 nerve cells called the suprachiasmatic nucleus, or SCN. Think of the SCN as the master clock.

Its main cue is light. When light hits specialised cells in your retinas — even with your eyes closed — they send a signal straight to the SCN. The SCN then tells your body: “It’s daytime. Be alert.” It suppresses melatonin, raises cortisol, and increases body temperature and heart rate.

When darkness falls, the SCN flips the switch. The pineal gland releases melatonin, your “sleep hormone”. Body temperature drops, metabolism slows, and you get drowsy. This is why screen time at night, or blackout curtains in the morning, can throw you off.

What circadian rhythms regulate

Sleep is just the most obvious output. Your circadian system also times:

Hormones: cortisol peaks around 8 am to wake you up. Growth hormone surges during deep sleep to repair tissue.
Metabolism: insulin sensitivity is higher in the morning. Late-night eating fights your biology, which is one reason shift work is linked to metabolic disorders.
Cognition: alertness, reaction time and memory peak in the late morning and dip in the early afternoon — that 2 pm slump is real.
Body temperature: lowest around 4 am, highest in the late afternoon. This cycle helps you fall asleep and wake up.

When rhythms are disrupted

Your clock is persistent, but not unbreakable. Three common disruptions:

Jet lag: flying across time zones forces your SCN to reset. It usually adjusts by about an hour a day, which is why travelling east feels harder.
Shift work: working nights forces your body to be alert when it wants to sleep. In the long term, this is linked to higher risks of diabetes, heart disease and depression.
Social jet lag: staying up late at weekends and waking early on weekdays. Your body gets conflicting signals and never fully settles.

Even small changes matter. The single hour of sleep lost when clocks move forward for daylight saving time correlates with measurable spikes in heart attacks and traffic accidents.

Keeping your clock healthy

You can’t replace the SCN, but you can anchor it. This is called “circadian hygiene”.

Light is the strongest lever. Get bright light, ideally sunlight, within an hour of waking. Dim the lights 2–3 hours before bed. Blue light from phones especially mimics the morning sun to your SCN.
Keep consistent sleep and wake times. Your body loves predictability more than it loves 8 hours. Sleeping from 12 midnight to 8 am every day beats 10 pm to 6 am on weekdays and 2 am to 11 am at weekends.
Time your meals. Eating within a 10–12 hour window during daylight reinforces the clocks in your liver and gut. Late dinners delay them.
Temperature. A cool bedroom, around 18 °C, helps the natural dip in temperature that triggers sleep.

Circadian biology won a Nobel Prize in 2017, for good reason. Almost every cell has clock genes that switch on and off in 24-hour cycles. When we fight those cycles, performance, health and mood all suffer. When we work with them, sleep improves, learning sticks better, and the long-term risk of disease drops.

We used to live by the sun. Now we live by screens and schedules. Your circadian rhythm is the old code still running underneath. Sync with it, and most of your body’s systems run more smoothly for ever.