You’ve likely heard people say, “No screens before bedtime.” Or perhaps you’ve been told of the importance of getting enough sleep. Maybe you’ve been taught to eat breakfast right after waking up. But what’s behind all this advice? Your circadian rhythm tells your body when to sleep and when to wake, keeping you in sync throughout the day. Almost all living things follow a 24-hour pattern. Circadian comes from the Latin for “about a day.” This makes sense, as the circadian rhythms of most adults are roughly a day long.
Your circadian rhythm is affected by more than just your bedtime habits. It also shapes your body’s functions, such as digestion, sleep patterns, and even how your brain works. So, whether you’re a night owl or an early bird, keeping your circadian rhythm in sync, day and night, can help you improve your health and well-being.
How circadian rhythms work
Over millions of years, our circadian rhythms have lined up with the Earth’s day and night cycles. These rhythms are influenced by internal clocks in many organs and glands and led by a master clock in the brain. Like the Earth’s rotation, this master clock follows a 24-hour cycle, changing by about 12 to 18 minutes each day.
Your master clock syncs with this light-dark cycle through signals from the eyes, making light the strongest cue for your circadian rhythm. Other cues, such as hormones and feedback from the autonomic nervous system , also inform your body’s daily rhythm. These internal and external cues shape your sleep-wake cycle, determining when you feel awake and when you need sleep.
Your circadian rhythm helps your body respond to the many shifts around you each day by helping to regulate:
Body temp. Humans have better recall, attention, and mental sharpness when their core body temp is higher. Changes in body heat can impact sleep-wake cycles. For example, cooler temperatures at night support sleep, while warmer temperatures can upset it. Each day, your body adjusts with your circadian rhythm, meal plan, and exercise to sync with your core body heat.
Digestion. Circadian rhythms help control when digestive enzymes and stomach acid are made, making your body better at breaking down food during the day. They also control the release of hormones that make you feel hungry or full.
Hormone levels. Cortisol and melatonin also play important roles in your circadian rhythm. Cortisol, the stress hormone, peaks in the morning to wake you up and then lowers later in the day. Melatonin, the sleep hormone, rises when the sun starts setting and peaks for most people from 2 to 4 a.m. during the deepest stages of sleep.
Immune system function. Circadian rhythms help control the immune system by deciding when immune cells are made and how active they are. White blood cells make sure the body can fight infections at the right times. Certain proteins that follow your circadian rhythms are key to healing muscles and tissues. So, if immune cells’ rhythms are disrupted for too long, you may be more likely to get sick or develop health problems with your organs.
Mental and physical performance. Based on hormones and light, a normal circadian rhythm follows this pattern: Clear thinking and focus peak in the late morning. The fastest reaction times happen between 2 and 4 p.m. Muscle strength has been found to improve around 6 p.m. Sex drive often spikes in the early morning and around 10 p.m.
Metabolism. Your circadian rhythm regulates metabolic processes like when to expend energy and when to store it during the sleep/wake cycle. Because of this, the body can better process glucose during the day.
What affects your circadian rhythm?
Your body relies on external cues to stay in tune with the world you live in. The two key factors that change your circadian rhythm are light and darkness. These cues, or lack of them, tell your body what time it is. There are also many other circadian rhythm cues called “zeitgebers,” which is German for “timekeepers.” Circadian timekeepers include:
Sunlight and man-made light: Natural light is the main cue for setting circadian rhythms. Contact with morning sunlight helps sync the body’s clock with the 24-hour day, helping you feel awake and alert. Man-made light—mainly blue light from screens—can disturb circadian rhythms by slowing melatonin production. This makes it harder to fall asleep and sleep well. It also tricks your body clock into thinking it needs to reset back to daytime.
Food intake. Eating during daylight hours tells the cells in your organs, like the liver and heart, to sync with your brain’s master clock. This helps boost metabolism and energy. However, eating at irregular times can disrupt these circadian rhythms. For example, eating during the resting phase can lead to weight gain as your metabolism slows down when you sleep.
Exercise. Moving your body daily can help even out circadian rhythms and help sleep quality. It also boosts melatonin at the right times, which adds to time spent in deep sleep and helps you relax. Plus, exercise supports mood, which can further improve sleep quality and keep circadian disruptions at bay.
Stress. When your body is under stress, it releases hormones and chemicals that speak to the body’s master clock. When not released in sync with a normal circadian rhythm, these can disrupt your sleep-wake cycle and your body systems.
Some drugs. Heart meds like alpha-blockers and beta-blockers may reduce deep REM sleep and lower melatonin levels. Antidepressants can also lead to insomnia. Cholesterol meds and Alzheimer’s drugs may disturb sleep with muscle pain and bad dreams. Talk to your doctor before trying melatonin supplements or sedatives.
Circadian rhythm disorders
Circadian disorders can happen when the sleep-wake cycle is revised, leading to various health issues. There are many illnesses that can be linked to circadian disruption. Here are the most common:
- Night shift work
- Artificial light/blue light
- Jet lag
- Changes in bedtime
These issues can lead to short-term signs of tiredness, hormone shifts, and digestion issues. Circadian disruptions can also lead to long-term health issues that change body systems such as your heart, metabolism, and nervous system. Sometimes disruptions can lead to sleep issues and vice versa. These include:
- Jet lag disorder
- Advanced sleep-wake phase disorder
- Irregular sleep-wake rhythm disorder
- Delayed sleep phase syndrome
- Shift work sleep disorder
As people age, the rhythm and timing of their body clocks decline. Neurons in the brain that promote sleep are lost as part of normal aging. Conditions like Alzheimer’s can speed up this loss. The drop in neurons makes it harder for older adults to stay asleep. In fact, people over 65 spend much less time in the deep sleep stage. So older adults tend to sleep less and wake up earlier.
How to reset and maintain your circadian rhythm
If your circadian rhythm gets altered, don’t lose hope! Whether you stayed up binging a show for a few nights or just have trouble falling asleep, here are a few steps to help reset your circadian rhythm:
- Get the right light at the right times. Get out in the natural light first thing in the morning and throughout the day. If you’re unable to get natural light, you can try a light box. Sometimes known as a light therapy box, this device emits bright light and is thought to help regulate your body clock.
It’s important to shut off all computers, television, and other screens at least one hour before going to bed. Screens have blue tones, so stick to warm lighting from lamps or even candles. If you must look at a screen, invest in blue light-filtering glasses. Also, consider switching your screen settings to the lower light “nighttime” setting. Finally, to ensure your rhythm is set for rest, it’s best to sleep in total darkness. - Eat at consistent times. When you eat is just as important as what you eat. Make sure to eat regular nutrient-dense meals during the day. Start your day with a healthy breakfast and don’t snack too close to bedtime. But if you have night sweats, shakiness, or other signs of low blood sugar while sleeping , it's okay to eat a healthy snack before bed. Pay attention to your physical hunger cues but try not to let cravings get the best of you.
- Move your body during the day. Exercise boosts mood, reduces stress, and helps deep sleep. Morning aerobic or strength workouts wake up your body and may stimulate melatonin release later. High-intensity workouts are best in the afternoon, while light evening workouts can help you sleep better.
Note that working out raises body temperature. To give your body time to cool down, exercise in the morning or at least 90 minutes before bed. - Follow healthy routines. Maintaining regular daily routines helps reinforce circadian rhythms . Try to go to bed and wake up at the same time each day, even on weekends. Practice healthy morning habits that give you energy. Reduce stimulation before going to bed to support a healthy sleep/wake cycle so your sleep is more restful.
It also helps to practice sleep hygiene . Create a bedtime routine with 3 to 4 relaxing habits. Try stretching, deep breathing, and reading for 30 minutes. A warm bath about an hour before bed can also calm your body and trigger a quick cooldown, which tells your circadian clock it’s time to sleep.
Your circadian rhythm is more than just your sleep-wake cycle. It influences everything from what time you wake up to how well your body heals. Living an active, healthy lifestyle can help you stay in sync with your circadian rhythm, which in turn helps you maintain healthy habits.
If you’re struggling to fall asleep or wake up, or if you have persistent fatigue, talk to your health care provider. Paying attention to your body’s rhythms can help you function at your best, both day and night.
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This article was written by Celina Johnson, edited by Kimberley Reynolds, and clinically reviewed by Elizabeth Thompson, MPH, RDN, on August 20, 2024.
