It’s the same story every night: You finish dinner, put your plate in the sink, and then dive into your evening activity du jour. Maybe you settle on the couch in front of the TV or with a good book, or perhaps you spend some quality time bonding with your loved ones. But as the sky goes dark, you find that you can barely keep your eyes open. Why does it feel like they always get heavy at the exact same time?

Blame your circadian rhythms, a series of clocklike systems in your body. As SleepFoundation.org describes them, “circadian rhythms work by helping to make sure that the body’s processes are optimized at various points during a twenty-four-hour period.” In other words, they regulate your energy throughout the day, gear you up for high-energy activities like exercise and work, and calm you down for sleep at night.

So why does your brain do this? The human body craves consistency. No matter your unique behaviors and ever-changing tastes, your brain wires you to follow a repetitive routine, ensuring your bodily functions are attuned to all your daily needs.

Moving to your rhythm

Your body’s internal clock is an amazing feat of nature. It adjusts your energy based on a cycle that runs very close to twenty-four hours—in fact, it’s so accurate that it can wake you up right before your daily alarm goes off, almost down to the minute. Besides sleep, this system is also responsible for other functions like hunger, digestion, and hormone regulation.

The command center of these functions is the suprachiasmatic nucleus (also called the circadian pacemaker), which is located in the structure of your brain known as the hypothalamus. The hypothalamus coordinates communication between your nervous system and your endocrine system, triggering the latter to produce hormones. When your circadian pacemaker senses changes in the environment, such as sunrise and sunset, it sends signals throughout your body to react accordingly.

Your circadian pacemaker is highly sensitive to shifts in light. When nighttime falls, it prompts the pineal gland in your brain to produce and release melatonin. This hormone calms your body’s continuous processes, slowing your breath and heart rate, lowering your body temperature, and, in turn, enabling sleep. In response to daytime light, melatonin production slows until you eventually wake up. This is why early risers can feel alert and energized as soon as the sun comes up.

Rhythm for everyone

This fascinating natural clock isn’t found only in humans. Other life forms have circadian rhythms of their own. This biological feature tells flowers when to bloom or close and warns nocturnal animals not to leave their shelter during the day. It even reacts to changes in the length of night and day throughout the seasons, which activate the hibernation response in some wildlife and provide seasonal information to migrating birds.

However, there is one aspect seemingly unique to humans: our circadian rhythms can become inaccurate or inconsistent. Problems with this natural flow can make healthy sleep a challenge and even interfere with daytime responsibilities.

Circadian rhythm disorders

Unfortunately, a person’s inner sleep-wake clock isn’t always in sync with their lifestyle or rest needs. If you feel that no matter what you do, you just can’t fall asleep, you may have a circadian rhythm disorder. This occurs when your cycle is unaligned with environmental cues such as sunrise and sunset, leading to poor or unfulfilling sleep.

There are several different types of circadian rhythm disorders, some of which are impacted by biological factors like age and genetics. For example, many middle-aged and older adults suffer from advanced sleep phase disorder, which causes them to fall asleep very early and wake up hours before sunrise. Conversely, young adults often struggle with delayed sleep phase disorder (SPD), a condition that leads them to sleep and wake about two hours later than the average adult. Because few social systems allow for such a sleep pattern, many SPD sufferers have difficulty adhering to early-morning school and work demands.

General symptoms of a circadian rhythm disorder may include struggling to fall asleep, grogginess when waking up, and difficulty engaging in everyday activities like work and exercise. In extreme cases, you may even experience severe daytime fatigue, trouble staying alert, memory problems, or cognitive issues. Left untreated, these symptoms can increase your risk of dangerous workplace or driving accidents.

What throws off your sleep cycle?

Sometimes, your own habits may be to blame. Behaviors that can override your circadian rhythms include:

• Varying your sleep schedule dramatically. This is why people who enjoy late weekend nights experience grogginess and low motivation on Monday mornings.
• Traveling outside of your normal time zone, especially if sunrise and sunset occur at drastically different times than you’re used to. It may take a few days for your body to adjust to this new cycle.
• Using electronics before bed. Studies have shown that your brain perceives bright, stimulating light from devices like cell phones as a cue to stay awake, not wind down.
• Depending too much on caffeine. While caffeine can help you overcome daytime drowsiness, excessive or late-day consumption may interfere with the onset of sleep.
• Working late at night or overnight. This can cause shift work disorder, which may induce fatigue and insomnia.

Correcting an offbeat rhythm

Circadian rhythms are so sensitive to your habits that you may be able to correct an improper sleep cycle on your own by creating a consistent routine. However, certain types of sleep disorders, such as sleep apnea, may require professional treatment. See a doctor to address such concerns, as untreated sleep issues can eventually lead to serious health problems and interfere with your work life and relationships—not to mention leave you staring at the ceiling every night.

For more info, visit sleepfoundation.org