Everything You Need to Know About Circadian Rhythms and How They Affect Sleep

You probably know that having poor sleep habits is bad for your health and well-being. But did you know that sleeping at the wrong time of day might do just as much harm?

Chronobiology is the study of the body’s circadian rhythms, and it suggests that the daily schedule of when we do things like eat and exercise — and especially when we go to bed and when we’re exposed to light — may play a vital role on day-to-day bodily functioning, and ultimately it may be connected to whether we develop longer-term health conditions such as diabetes or cancer.

Here's how our circadian rhythms work. They're the natural physical, mental, and behavioral changes that happen in the body, and they follow a 24-hour cycle, usually in response to lightness and darkness, according to the National Institute of General Medical Sciences. One example is being awake during the daytime and sleeping at night.

When it comes to human sleep, our biological rhythms, or internal body clocks, are our cells’ natural timing devices. They’re a collection of molecules that interact with cells to tell them when to do certain things (like release hormones that make you feel sleepy, awake, or hungry) that keep those circadian rhythms on track.

The presence of one set of molecules triggers the production of other molecules, which in turn triggers the next phase in the cycle and enables cells to keep time, explains Kenneth Wright Jr., PhD, a sleep and chronobiology researcher at the University of Colorado in Boulder.

They body also has one master internal clock, Dr. Wright says. That clock is housed in the brain’s hypothalamus (above the optic nerve); it’s called the suprachiasmatic nucleus, and it contains specialized cells that use a molecular process to keep time, research has shown. (It’s also sometimes called the body’s master circadian clock or circadian clock, according to the Centers for Disease Control and Prevention.)

The master clock takes its cues from the lightness or darkness we’re exposed to (the fact that these cells live near the eyes is not a coincidence — the cells receive direct input from the eyes). And the master clock coordinates the body’s other clocks by regulating things like body temperature and hormone levels, which in turn trigger the molecular clocks in individual cells, Wright says.

Most bodies run on similar schedules (in terms of naturally wanting to sleep at night and be awake during the day) because we’re exposed to similar light patterns over the course of the day. But there’s variation based on our behaviors, certain genetic components, and other factors.

Research has identified, for instance, that certain genes are linked to whether or not someone has an earlier or later circadian rhythm (if they’re an early bird or a night owl), explains Michael Breus, PhD, a clinical psychologist and author of Energize! and The Power of When, both about chronobiology and optimizing daily activities based on the body’s internal clock.

Some people are right in the middle, and others may tend to naturally lean toward one end of the spectrum or the other: night owls and early birds. Others may have dysregulated circadian rhythms (and sleep patterns) because of other factors, like a nighttime work schedule or a sleep condition like insomnia, Dr. Breus says.

The brain will try to sync its clock to daylight, so when you’re exposed to artificial light late at night, it can push your biological clock toward the night-owl end of the spectrum, Wright adds.

(There are also circadian rhythm disorders, such as delayed sleep phase disorder and advanced sleep phase disorder. These are sleep problems that involve a disruption in the timing of sleep, which can be caused by a master clock malfunction of some kind or some external factor that misaligns your daily patterns, according to Cleveland Clinic.)

Age is another factor that can affect circadian rhythms and whether the natural sleep-wake cycle (and other cycles) tends to run early or late, Breus says. Teenagers, for example, may need a later wake time and may be more alert later in the day. Then, when we become adults, the sleep cycle moves back toward the middle of the day again, research shows. Older adults may have a harder time falling asleep and wake up earlier in the morning, according to MedlinePlus.

Because our internal clocks, which each control various bodily functions, are linked to the master body clock in the brain, lots of things can go wrong if our circadian rhythms get thrown off schedule. The sleep-wake rhythm is a key one because physiological processes such as the production of appetite hormones, digestion, and even the immune system are designed to activate when we’re awake and slow down when we’re asleep.

“Circadian rhythms themselves are really fundamental to our biology,” Wright says. “If we disturb them, it has an impact on most systems.”

For example, the digestive system has its own circadian rhythm. After we wake up, the body releases certain hormones that make us feel hungry (and prompt us to eat) and other hormones that help us break down and digest that food. These processes slow down during sleep.

When we eat too early or too late in the day, fewer of those hormones that help with digestion are available, so the body has a tougher time regulating blood sugar after eating and absorbing and storing nutrients from that food. That’s why, over time, those types of behaviors are thought to potentially contribute to the development of obesity and diabetes, some research has suggested.

If circadian clocks within individual cells become sufficiently disturbed, Wright says, research suggests it may, in some cases, contribute to those cells becoming cancerous — if, say, cells begin to divide at inappropriate times.

Research that Wright worked on has shown that when healthy people shift their sleep-work schedule for just two nights (sleeping during the daytime hours and working overnight), it can alter more than 500 proteins known to play a role in the development of (or prevention of) a host of chronic health conditions, from cancer to immune disorders to metabolic disorders. “Just having people sleep during the day changes a lot of our fundamental physiology,” Wright explains.

Research suggests that circadian rhythm disruption and misalignment may play a role in the development or progression of the following health issues:

• Diabetes
• Heart disease
• Some cancers
• Depression

Getting less sleep than is healthy for you (less than seven hours per night for adults) and significantly shifting your sleep schedule (sleeping in several hours later on weekends, compared with weekdays, or traveling across time zones) are among the most common disruptions to your body's clock.

But other factors can disrupt circadian rhythms, too.

Exposure to sunlight is a big one. The natural light dark cycle greatly affects your body clock. Some people who are blind, for instance, have disrupted circadian rhythms because they’re not getting that light input to the master clock in the brain. Research suggests that this can lead to a condition where the body fails to sync to the environment and begins “drifting,” causing them to experience extended periods of what feels like jet lag, Wright says.

An overnight work schedule is another factor that’s for problematic circadian rhythms, even if you are getting an adequate amount of sleep, Wright says.

Spending too much time indoors under artificial lights, particularly later in the day, may also disrupt the body’s natural rhythms, pushing you toward a later bedtime, as past research that involved young adults suggests. And subsequently, you may find it more difficult to wake up the next morning because of that bedtime shift. (The body’s circadian cycles are not designed for late-night TV binges.)

Eating habits, such as having dinner or a large snack too close to bedtime, can send signals to the brain to rev up digestion and therefore stay awake, Wright says. When you exercise, it can send signals in the body that affect circadian rhythms, too, research suggests.

Long-term use of certain drugs, like caffeine, melatonin, or marijuana, may impact your circadian rhythms as well, Breus says.

Maintaining a consistent sleep schedule — even on the weekends — is one of the biggest ways to keep your circadian clock on track, Wright says. When that doesn’t happen, follow these tips to get your circadian rhythms back on a healthy schedule, and continue these habits to keep the clocks running on time:

• Avoid eating and physical activity later in the day. This is especially important in the two hours before going to bed, Wright says.
• Reduce exposure to artificial light at night. In particular, try to minimize time spent in harsh, bright lighting (like that from fluorescent bulbs or the blue light from your cellphone or computer). Use lamps with soft lighting in your home, and if you need to use your devices, shift them to a setting where blue light is muted.
• Get outside in the morning. Or expose yourself to plenty of other bright light in the morning. Natural light via sunlight is ideal, but artificial light is better than none. If you work indoors, Wright recommends trying to set up your desk near a window, if possible. If not, he suggests taking your morning coffee outdoors first to get some morning sun exposure.
• Ask your doctor about melatonin supplements. Melatonin is a naturally occurring hormone produced by the body to help promote sleep at night. In some cases, such as before or after traveling across time zones or for short-term bouts of insomnia, low doses of melatonin supplements (1 to 3 milligrams) can help the body readjust to a healthy sleep-wake schedule, according to Johns Hopkins Medicine. The supplement can interact with certain medications and shouldn’t be used long term, so it’s a good idea to talk with your doctor about trying it first.

If you do shift work or work other irregular hours, you may not be able to avoid daytime sleep and nighttime work. Those people should pay particularly close attention to trying to prioritize other healthy choices, like getting enough physical activity and not smoking, because those healthy habits may decrease the health risks associated with overnight work, Wright says. And if nighttime workers are able to keep their mealtimes to daytime hours, or at least more closely aligned with a typical eating schedule, that’s ideal, Wright adds.

Travel can also throw off your sleep schedules and the body’s circadian rhythms. Wright recommends slowly adjusting your sleep schedule by going to bed and getting up earlier before traveling into a new time zone, so that it’s not as much of a shock to your system when you go. Adjust your sleep schedule by one hour per day (starting enough days ahead that you will be on the schedule of your arrival destination by your departure date), in order to reduce jet lag upon arrival.

If you’re having particular difficulty sleeping or maintaining a sleep schedule, Wright recommends getting in touch with a sleep medicine specialist or even a circadian medicine doctor who can help you make needed lifestyle changes and detect underlying conditions that may impact your ability to regulate your circadian rhythms.