Circadian Rhythms: What Are They & How to Improve Yours

Your circadian rhythm, also known as your body clock, is responsible for a whole host of bodily functions. One of its most important roles is the way it shapes your sleep-wake cycle. Here, we look at what the term means and how it affects the way you sleep.

Circadian rhythms are the roughly 24-hour cycles that make up your body’s internal clock. They influence far more than sleep, from your body temperature and hormone levels to your appetite, but their best-known job is running your sleep-wake cycle, the rhythm that wakes you in the morning and makes you sleepy at night.

Circadian rhythm definition: any regular variation in your body’s physical or behavioural activity that repeats on a roughly 24-hour cycle, such as the sleep-wake cycle. Source: APA Dictionary of Psychology.

At the centre of it all is a small cluster of cells in the brain called the suprachiasmatic nucleus, your master clock. It takes its main cue from light. When morning light reaches your eyes, it tells your brain to wind down melatonin and feel alert. As darkness falls, melatonin rises again, and you start to feel sleepy. This is why light, more than anything else, keeps your body clock on track.

Circadian isn’t the only biological rhythm. There are three others: diurnal, ultradian and infradian.

• Diurnal rhythms are closely tied to the circadian rhythm and centre on day and night. For sleep chronotypes who don’t follow a typical routine, their diurnal pattern looks different from the norm.
• Ultradian rhythms run over much shorter periods and repeat several times a day, like the roughly 90-minute sleep cycle you move through each night.
• Infradian rhythms stretch beyond 24 hours, covering things like the menstrual cycle and seasonal shifts in mood or appetite.

Plenty of things can knock your circadian rhythm out of sync. If you’re struggling to keep a sleep routine, it’s worth looking at the usual culprits.

Light at night

Light is your body clock’s main signal, so the wrong light at the wrong time confuses it. Screens are the common offender, with phones, laptops and tablets tricking your brain into thinking it’s still daytime. Over time, mistimed light has been linked to disrupted rhythms and knock-on effects for mood.

Shift work

Working through the night asks your body to be awake when its clock expects sleep. This can tip into a recognised shift work sleep disorder, partly because melatonin only rises at night, so daytime sleep tends to be lighter and shorter. Our guide to melatonin explains more.

Social jet lag

You don’t need to fly anywhere to feel jet-lagged. Social jet lag is the gap between your body clock and your social schedule, like staying up late at weekends, then dragging yourself up on Monday. The term was coined by researcher Till Roenneberg, and large studies have linked this kind of circadian misalignment to poorer sleep. The fix is balance. The odd late night is fine, just try not to sleep into the afternoon to make up for it, since that pushes your clock further out.

Travel and jet lag

Crossing time zones leaves your body clock running on the time you left behind. Jet lag eases as your rhythm catches up, usually around a day per time zone. Our guide on how to stop jet lag has the practical steps.

Diet

What you eat plays a part, too. Caffeine and alcohol are well-known sleep disruptors, but some foods contain nutrients linked to sleep, like tryptophan, magnesium, calcium and vitamin B6, which your body uses to make serotonin and melatonin. Here’s a quick starting point:

 
There’s more in our guide to sleep foods.

A body clock that’s regularly out of sync affects more than your sleep. Ongoing disruption has been linked to metabolic problems like weight gain and blood sugar issues, and researchers have connected long-term disruption of the body’s clock genes to a higher risk of various health conditions. Even without a fully sleepless night, chronic mistimed sleep can dull your concentration and slow your reactions. Your rhythm also shifts as you age, which is part of why many older adults wake earlier. One off night won’t do any harm. It’s the steady pattern that’s worth keeping an eye on.

When the body clock is persistently misaligned, it can tip into a circadian rhythm sleep disorder. These include shift work disorder, jet lag disorder and delayed or advanced sleep phase, where your natural sleep timing sits much later or earlier than you’d like. If your sleep timing feels stuck and it’s affecting your days, it’s worth speaking to your GP.

The good news is your body clock responds well to a few consistent habits. Morning daylight, regular sleep and wake times, dimmer evenings and easing off late caffeine all help nudge it back into line. We’ve gathered the full set of steps in our guide on how to reset your body clock, and for more general ideas, there are ways to sleep better at night.