If you’ve ever noticed that you tend to feel energized and drowsy around the same times every day, you have your circadian rhythm to thank. What is it, exactly? Your circadian rhythm is basically a 24-hour internal clock that is running in the background of your brain and cycles between sleepiness and alertness at regular intervals. It’s also known as your sleep/wake cycle.

For most adults, the biggest dip in energy happens in the middle of the night (somewhere between 2:00am and 4:00am, when they’re usually fast asleep) and just after lunchtime (around 1:00pm to 3:00pm, when they tend to crave a post-lunch nap). Those times can be different if you’re naturally a night owl or a morning person. You also won’t feel the dips and rises of your circadian rhythm as strongly if you’re all caught up on sleep. It’s when you’re sleep-deprived that you’ll notice bigger swings of sleepiness and alertness.

A part of your hypothalamus (a portion of your brain) controls your circadian rhythm. That said, outside factors like lightness and darkness can also impact it. When it’s dark at night, your eyes send a signal to the hypothalamus that it’s time to feel tired. Your brain, in turn, sends a signal to your body to release melatonin, which makes your body tired. That’s why your circadian rhythm tends to coincide with the cycle of daytime and nighttime (and why it’s so hard for shift workers to sleep during the day and stay awake at night).

Your circadian rhythm works best when you have regular sleep habits, like going to bed at night and waking up in the morning around the same times from day to day (including weekends). When things get in the way, like jet lag, daylight savings time, or a compelling sporting event on TV that keeps you up into the wee hours of the morning, you can disrupt your circadian rhythm, which makes you feel out of sorts and can make it harder to pay attention . 

Interestingly, your circadian rhythm will likely change as you get older. And you may not have the same sleep/wake cycle as your partner, child or parents. But the more you pay attention to your body and notice feelings of alertness and drowsiness, and the more time you spend developing good sleep habits, the better your slumber will be and the better you’ll feel.