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Your body clock controls more than you think — sleep expert explains how circadian rhythm works
If you’ve ever wondered why you can’t nod off at night it could be down to your body's internal clock - also known as the circadian rhythm.
But what is the circadian rhythm? We all work on a 24 hour clock and your body is no different, with this internal clock helping to regulate hormones such as melatonin and cortisol.
Factors such as stress, diet and how much daylight you do or don’t get can dysregulate your circadian rhythm, leading to sleep problems such as daytime napping and not being able to nod off at night.
For Sleep Awareness Week 2025, we look at the latest research and ask Dr Leah Kaylor, a licensed clinical psychologist for the Federal Bureau of Investigation who specializes in sleep, what happens when your circadian rhythm gets disrupted and, more importantly, how to reset it through light exposure, meal timing, and lifestyle adjustment so you can reclaim a good night's sleep.
What is Circadian Rhythm?
Ever wondered what makes you wake up in the morning or start yawning by the afternoon?
Your Circadian Rhythm is your body’s internal 24-hour clock that not only controls your sleep, but also other important parts of how your body functions.
“It regulates sleep-wake cycles, hormone release, body temperature, digestion, and other biological processes,” explains sleep expert Dr Leah Kaylor. So why does this happen?
“The circadian rhythm is primarily controlled by the suprachiasmatic nucleus (SCN) in the hypothalamus, which responds to light and darkness to keep your body synchronized with the day-night cycle.”
What does your Circadian Rhythm do?
Okay, we covered the basics, but what does daylight and night time have to do with our sleep cycle?
Melatonin is the hormone that helps the brain keep track of time, specifically what time of day we should be asleep and awake. Low levels of melatonin will make you feel more awake, whereas high levels will make you feel sleepy.
It’s all to do with hormones, says Dr Kaylor: “When natural light enters your eyes, the SCN signals the brain to suppress melatonin (the sleep hormone) and increase cortisol, helping you feel awake.”
She adds that during the day you are active and alert so your metabolism, body temperature, and cognitive functioning are at their peak. However, as the sun sets, melatonin begins to rise, preparing your body for sleep.
“As you prepare for bed, the body cools down, your heart rate slows, and growth hormone is released to support cell repair and immune function.”
What affects Circadian Rhythm?
Of course none of us live the same type of life, with different stresses and lifestyle habits, so this is how some of those factors could affect your circadian rhythm.
1. Stress
We all know too much stress can put a big strain on your body, while a study in 2018 showed that stress can actually override how your circadian rhythm works.
“Chronic stress keeps cortisol levels high, delaying melatonin release and making it harder to fall and stay asleep,” Dr Kaylor explains.
“Stress also activates the sympathetic nervous system (‘fight or flight’), leading to more nighttime awakenings and lighter sleep.”
While she also adds that stress can shift your natural sleep-wake cycle, causing “delayed sleep phase syndrome (going to bed too late) or early morning awakenings.”
2. Light and dark
As you may have already gathered, light and dark can affect your sleep patterns. But, how can this get disrupted?
Not getting out into daylight during the day can trick your body into thinking it's nighttime, while even having too many lights on in the evening can suppress your night time hormones.
3. Physical activity
Any type of physical activity, running, walking and even gym classes can be great for your circadian rhythm — and research proves it.
This study from the University of Texas in 2023 shows that regular exercise promotes synchronization between the sleep-wake cycle and the circadian clock.
However, timing is key, says the sleep doctor: “Ideally exercise in the morning and bonus points if you can do it outside in the sun which will serve to strength the circadian rhythm.”
4. Diet
When and what we eat can not only affect our waistline but our sleep cycles too.
“Eating too close to bedtime will keep the body active and digesting making it more difficult to fall asleep as well as not being able to drop down into the deepest stages of sleep because digestion is a very energy intensive process.”
So when should you have your last meal of the day? “Aim to finish your last meal 2-3 hours before bed.”
While drinking too much coffee or tea can also have a negative effect. “Caffeine is a stimulant that works to keep the body awake,” Dr Kaylor says.
“Aim to cut off caffeine in the early afternoon. If you are sensitive to the effects of caffeine, cut it out even earlier,” she advises.
Signs of a dysregulated circadian rhythm
When your circadian rhythm is dysregulated, you're likely to notice a range of symptoms that can impact your day-to-day life.
Dr Leah Kaylor pinpoints the main symptoms of a dysregulated circadian rhythm to look out for:
- Trouble falling asleep at night or waking up too early
- Trouble staying asleep and frequent awakenings
- Feeling tired in the morning (even after a full night's rest)
- Daytime fatigue, brain fog and low energy
- Inconsistent sleep patterns which might look like early morning awakening for work then sleeping in on the weekends
- Reliance on naps or caffeine to get you through the day
How to regulate your Circadian Rhythm for better sleep
Get plenty of daylight in the morning
Work and life’s many stressors can disrupt us from our normal wellness routines, but experts advise to get out in daylight as soon as we can, and Kaylor agrees.
Exposing yourself to bright daylight within 30-60 minutes of waking will reset your body clock
Dr Leah Kaylor
“Exposing yourself to bright daylight within 30-60 minutes of waking will reset your body clock,” she says.
Even if this isn’t possible — maybe you’ve got a deadline or you're stuck inside with the kids — then just getting out before the sun goes down for a quick walk will help you sleep better later.
A study in 2019 proved the importance of getting as much daylight as possible, explaining that morning light helps to advance your internal clock, while evening and night light delays the clock.
Keep a regular sleep schedule
Sleeping in until 10am on a Saturday or Sunday may feel like a nice indulgence after a busy week, but it could also confuse your body’s internal clock.
So, what’s the answer? Kaylor explains that you need to be consistent with your sleep schedule.
“Go to bed and wake up at the same time every day—even on weekends. Do not sleep in on days off. This confuses the brain and makes Monday mornings that much harder.”
Create a nighttime routine
We’re all guilty of swiping or bingeing on our phones until gone midnight, but this can have a detrimental effect on our hormones.
The blue light emitted from our phones can stop the production of melatonin, meaning we're lying awake, instead of falling into a deep sleep. Instead, create a calming screen-free nighttime routine.
Start with a warm bath or shower because this will cause your core body temperature to drop as you move from a warm bathroom to a cooler bedroom, which is a key signal to your body that it’s time to sleep. Then, opt for relaxing activities like reading or listening to a podcast.
Start your routine at least 30 minutes before you intend to fall asleep so your mind and body have time to switch off and transition to a sleeping state.
Tom's Guide created this content as part of a paid partnership with Helix Sleep. The contents of this article are independent and solely reflect the editorial opinion of Tom's Guide.
