What Is Circadian Rhythm? Decoding the Body’s Sleep-Wake Cycle

Circadian rhythms are an essential element of the human sleep schedule. They determine when we feel sleepy, when we feel alert, and how often we wake throughout the night. While circadian rhythms are naturally occurring, they’re easily disrupted. Whether its blue light or a hormonal imbalance, both internal and external factors can influence the way our body regulates sleep.

So what is circadian rhythm, and why does it matter for sleep health? Read on to find out more about the importance of this natural physiological cycle.

What Is Circadian Rhythm?

Humans follow a 24-hour sleep-wake cycle, which tells us to sleep when it’s dark and stay awake during the day. This cycle is called the circadian rhythm, and it’s one of the essential biological processes for ensuring we get enough rest each night.

The circadian rhythm is driven by certain areas of the brain, explains the Division of Sleep Medicine at Harvard Medical School. For example, the brainstem and hypothalamus promote wakefulness by sending neurotransmitters, or chemical arousal signals, to the cerebral cortex. This keeps the cortex — the brain’s largest region — activated to keep you awake.

In contrast, neurons in a different part of the hypothalamus help a person feel tired by inhibiting these wakefulness messages. The neurons regulating your circadian rhythm work together harmoniously to promote a restful sleep schedule.

“The neurons that promote wakefulness inhibit those that promote sleep and vice versa. This interaction normally leads to either a relatively stable period of wakefulness or a relatively stable period of sleep,” adds Harvard’s Sleep Medicine Division.

Feeling sleepy at bedtime is also dependent on the timely release of melatonin, the hormone that regulates sleep. Melatonin is driven by external cues like sunlight and darkness. Without our circadian rhythm, we’d have an inconsistent sleep-wake cycle, and we might not get the rest we need to keep us energized throughout the day.

Factors That Influence Circadian Rhythm

Despite the fact that circadian rhythm is regulated by the sun, it’s also sensitive to lifestyle and environmental factors. In turn, throwing off your biological clock can lead to serious sleep deprivation, irritability, depression, fatigue, headaches, and a host of other ailments. If you feel like you can’t fall or stay asleep when you should — or waking up is a constant chore — these are a few things that might be getting between you and a good night’s sleep.

Circadian Rhythm Disorders

The first thing you’ll want to rule out is the possibility of a circadian rhythm sleep disorder. There are two main types: advanced sleep-phase syndrome (ASPS) and delayed sleep-phase syndrome (DSPS).

“Advanced sleep-phase syndrome is characterized by a persistent early evening sleep onset time (between 6:00 p.m. and 9:00 p.m.) and an early morning wake-up time (between 3:00 a.m. and 5:00 a.m.),” says doctor Mary E. Cataletto.

In contrast, delayed sleep-phase syndrome is when a person can’t fall asleep and awaken at socially acceptable times. Young adults and teens tend to experience DSPS more than older people, yet it can occur at any age.

ASPS is more common among older adults and people struggling with depression. While these people tend to get enough sleep, they’re not able to fall asleep and stay alert at an appropriate time of day.

Mental Illness

Circadian rhythm can also be influenced by mental illnesses such as bipolar syndrome. In fact, studies show that circadian rhythm disruptions are much more common in people with bipolar disorder than those with other major depressive disorders. This suggests that disruptions to the sleep-wake cycle are a hallmark sign of bipolar disorder and a way to differentiate it from other mental illnesses.

Seasonal affective disorder (SAD) is another depressive condition that can lead to circadian rhythm irregularities. Affecting about 5% of the population, people with seasonal affective disorder experience depression only during the late fall and winter months, when the days are shorter and periods of darkness are longer. Reduced exposure to sunlight may trigger a chemical imbalance in the brain, which can subsequently influence both mood and sleep cycle.

Sleep and Work Schedule

If you’re a self-proclaimed night owl or you work the graveyard shift, your body clock is likely to suffer. Holding a job at non-traditional hours may make you more susceptible to shift work sleep disorder, which is characterized by excessive sleepiness and an inability to concentrate or stay energized during the daylight hours.

Experts believe that 10-40% of shift workers struggle with this circadian rhythm disorder. In addition to making a person more tired, people who experience shift work disorder may become more prone to accidents. From a mental health perspective, they can have poor coping and interpersonal skills or struggle with mood swings.

How to Reset Your Circadian Rhythm

Whether you pulled an all-nighter or you just traveled abroad, we all experience sleep disruptions from time to time. Here are the best science-backed tips for resetting your circadian rhythm and getting back on track.

Follow Good Sleep Hygiene

Good sleep hygiene is essential to maintain a proper sleep-wake rhythm. As explained by the American Sleep Association, practicing good sleep hygiene means making healthy changes to your daily schedule in order to promote restful sleep.

One example of good sleep hygiene is setting boundaries with technology in the bedroom. Since the blue light from electronics can disrupt your circadian rhythm, you might decide to stop using smartphones and tablets a few hours before bedtime. Additionally, you might decide to remove televisions, computers, and even smartphones from the room during sleep to help you create a more restful sanctuary.

This is also important because it helps you associate your bedroom only with sleep. When you’re constantly working on a laptop or scrolling through Facebook while lying in bed, your body will begin to associate that space with wakefulness, rather than sleep.

Time Eating and Exercising

Eating or exercising too late into the night can keep you awake and disrupt your circadian rhythm. Specifically, too much cardio too late into the evening can trigger endorphins that keep you awake and alert for a few hours. That’s why it’s a good idea to end any intense physical activity at least two hours before bedtime.

Instead, try guided relaxation or gentle stretching to calm your mind and send signals of sleepiness. Likewise, eating late means you’ll be digesting late, and since digestion requires the body to create energy, this too can keep you awake. Try to have your last meal at least three hours before bedtime.

Use Bright Light Therapy

If you’re struggling with seasonal affective disorder, bright light therapy can help get your circadian clock back on track. Since sunlight exposure is what regulates your circadian rhythm, exposure to artificial light can help a person get back to a normal sleep schedule. This is especially useful in places far north of the equator where long periods of darkness persist during the winter months.

“Light therapy is used to expose your eyes to intense but safe amounts of light for a specific and regular length of time. In many places, sunlight is not available at the proper time to be used as treatment,” says the American Academy of Sleep Medicine.

Maintain Sleep Cycles While Traveling

If you’ve ever taken a plane to a different time zone, you know how disorienting it can be. Desynchronosis — more commonly known as jet lag — is a circadian rhythm disorder that can cause fatigue, insomnia, anxiety, irritability, and a number of other physical symptoms. To prevent the onset of jet lag and ensure it doesn’t throw off your sleep schedule, Clayton Sleep Institute suggests adjusting your sleep schedule gradually before you leave.

If you’re flying east, go to sleep and wake up one hour earlier each day for three days before departure. For west-bound trips, go to sleep and wake up an hour later each evening and morning “for as many time zones you will cross, or as many as you can manage given the distance you are traveling,” says the institute.

Regardless of how much you’ve prepared for the time-zone change, do your best to get on the local time as quickly as possible to reduce potential sleep challenges. This will help you get better sleep and boost your overall well-being both during and after the trip.

Regulating Your Circadian Rhythm

Your circadian rhythm is the internal body clock that regulates sleep patterns. When your internal clock is disrupted by environmental or psychological factors, you’re likely to suffer from fatigue, irritability, and depression.

Fortunately, understanding how this 24-hour cycle works — and what mental health problems and sleep habits affect it — can get you back on the road toward deep sleep (and maybe even toward becoming a morning person).

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