The circadian rhythm – everything you need to know
Written byEmily Coates
Reviewed byStephanie Romiszewski
Your circadian rhythm shapes your sleep pattern, but what happens when it's out of sync? From melatonin to daily habits, we uncover the science behind your body clock with a sleep expert
Summary
1What controls the circadian rhythm?
Your circadian rhythm is controlled by the hypothalamus, which responds to light and signals the release of hormones like cortisol.
2Hormones that influence your sleep and circadian
Hormones including melatonin, cortisol and adenosine work together to regulate your sleep–wake cycle, helping your body feel alert during the day.
3How can I improve my circadian rhythm?
You can support your circadian rhythm by keeping a consistent routine, getting natural light during the day and limiting screen time before bed.
Did you know that 54% of adults say stress and anxiety were the top reasons for having poor sleep?¹ And women are 40% more likely to have insomnia than men?2 There are a lot of reasons why our sleep can be affected, but the answer may lie in more than just your lifestyle habits. Your circadian rhythm – or internal body clock – plays a key role in your natural sleep pattern.
We spoke to Expert Behavioural Sleep Therapist, Stephanie Romiszewski, to uncover everything there is to know about how your body clock works and what might be throwing it off.
What is the circadian rhythm?
According to Stephanie, “The circadian rhythm is the brain’s way of aligning the body with the environment. It’s a set of physiological changes that follow a roughly 24-hour cycle, guided by your internal body clock.”
“Our environment has an influence on this clock – most specifically light. This is why we sleep when it’s dark and we wake up when it's light. This natural cycle, sometimes referred to as circadian rhythmicity, helps regulate key processes in the body, including your sleep phases, energy levels and hormone release,” she says.
“Nowadays though, things like electricity, technology, alarms and commuting times mean that many of us are keeping hours which might be at odds with our natural body clocks,” she adds.
“Light stimulates the production of cortisol – which wakes your body up”
What controls the circadian rhythm?
Time to get your brain in gear for the science-y bit now… “Your circadian rhythm is controlled by a small area within the section at the base of your brain called the hypothalamus.3 The hypothalamus instructs the body to release certain chemicals and hormones during each 24-hour period,” explains Stephanie.
“The timing of these releases is in-built into our body clocks – but they’re also partly influenced by external factors, like whether it’s light or dark. Light stimulates the production of cortisol – which wakes your body up and energises you for the day ahead."4
Hormones that influence your sleep and circadian rhythm
Your sleep–wake cycle, also known as your circadian rhythm, is regulated by a complex interplay of hormones and signalling molecules that tell your body when to feel alert and when to rest. These processes are controlled by an internal biological clock and are strongly influenced by external cues such as light and darkness.5
One of the most well-known sleep-related hormones is melatonin. Melatonin is produced by the pineal gland in the brain and helps regulate the timing of your sleep–wake cycle.6 As light levels decrease in the evening, the brain signals the release of melatonin, which promotes feelings of sleepiness.5 In contrast, exposure to light, particularly at night, suppresses melatonin production, helping to maintain alertness.6
Rather than directly inducing sleep, melatonin acts as a biological signal of darkness, helping to align the body with the natural day–night cycle.6 Disruptions to light exposure, such as shift work, jet lag or late-night screen use, can interfere with this rhythm and make it more difficult to fall asleep.7, 8
Cortisol, often referred to as the 'stress hormone', follows a daily rhythm opposite to melatonin. It typically peaks in the early morning to promote alertness and gradually declines throughout the day to support sleep at night.
Adenosine builds up in the brain during waking hours as a by-product of energy use. This increases 'sleep pressure' and makes you feel more tired over time.9
Serotonin is a neurotransmitter involved in mood, but it also plays a role in regulating sleep. It acts as a precursor to melatonin, helping support the sleep–wake cycle.10, 11
Growth hormone is primarily released during deep sleep and plays a key role in physical recovery and repair.12 Together, the timing and balance of these hormones help maintain a consistent circadian rhythm, supporting healthy sleep phases, overall sleep quality and long-term health.
Why is the circadian rhythm important?
“It sets the pace for the body’s rest and repair. Deep sleep, for example, is vital for cell regeneration. So, whenever you’re catching good quality sleep, your body is using that time to restore itself," Stephanie explains.13
How do circadian rhythms differ?
“Just like people, no two circadian rhythms are the same and can also change throughout our lives,” says Stephanie. “When you’re a baby you sleep between 12-16 hours in a 24-hour period, reflecting the huge amount of growth that’s happening. Then when you’re aged one to five, you begin to synchronise with the light and dark circadian rhythm and should get between 10-14 hours, including naps."
National Institute of General Medical Sciences. Circadian Rhythms [Internet]. www.nigms.nih.gov. National Institute of General Medical Sciences; 2023.
Available from: https://www.nigms.nih.gov/education/fact-sheets/Pages/circadian-rhythms.aspx
What happens when circadian rhythms are disrupted, and what can disrupt it?
“Unnatural light and the timing of our activities,” says Stephanie. “This could be electric lights, a street light, the glare of a TV, mobile or other electronic devices. The blue light which comes from electronic devices can suppress melatonin production, which can stop you feeling tired, making it more difficult to drift off.”17
“Working shifts is another disruptor and can lead to insomnia and/or excessive sleepiness. It’s even been named Shift Work Disorder (SWD) by scientists."18
How to reset your circadian rhythm
“Firstly, give it routine and predictability,” explains Stephanie. “It's common for many people to believe that their early or late preferences are set in stone. This is true to a degree, but more often it’s our behaviours that ‘set’ our preferences. We have more control than we think.” Stephanie recommends to…
Get up at the same time every day
“See waking up as a type of reset button,” she says. “By waking at the same time every day you’re controlling the amount of sleepiness you’ll have built up by the end of the day."
Try to have a bedtime
“By going to bed when you’re sleepy or tired and then waking up at the same time no matter what, you’ll start to notice you become sleepy around the same time each night.”
Exercise during the day
“Physical activity can help improve sleep quality and the natural daylight you get by being outside helps synchronise your circadian rhythm."19
Try no phones in the bedroom
“Keep the bed for sleeping, not scrolling."
Try not to work late
“If you don’t have a natural separation between work and sleep, you’ll always be compromising on your sleep."
The final say
It all sounds simple enough, but putting it into practice is another thing. Try doing one thing at a time by habit stacking, as this should help you feel less overwhelmed and hopefully make you stick to the positive changes. If you’re looking for more ways to support your wind-down routine, discover feel-good finds and simple rituals to help you relax, refresh and recharge – wherever life takes you.
Disclaimer – This article provides informational advice and is not a substitute for medical care. Working with experts for accuracy, we take great care to ensure the information is up to date and relevant. However, you should always consult your GP or healthcare professional before using supplements or alternative products, particularly if you have a medical condition or are under supervision.
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