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Drowsiness isn’t the only consequence of an upset rhythm. Individuals who go against their internal clock “are most likely to develop things like metabolic disorders, obesity, diabetes, and cardiovascular disorders,” says Duffy.

And it’s surprisingly easy to get out of whack. Even just one night of staying up late can create cascading effects. “You might sleep a lot longer in the morning, but not necessarily because your clock is reset, but because you’re sleep deprived,” Duffy says. “By sleeping in, you miss the morning sun exposure that synchronizes your clock to the external world. And then that night, you don’t get sleepy as early.” And then a few days later, “it’s impossible to get up because you have shifted your clock,” she says.

Other common activities, like going on our phone before bed, can confuse the synchronicity, too, by mimicking natural light that we equate with daytime. Sleep is “very highly influenced by our behaviors and things that we do, especially our exposure to light,” Duffy says. “And so a misconception that people have is that if they don’t feel sleepy yet, they don’t need to go to bed yet.”

Quite a lot, actually. Circadian rhythms influence our immune systems, hormones, body temperatures, and metabolisms, and those are just some of the processes that have been studied. There might be many more systems that circadian rhythms govern that we haven’t even explored, says Duffy.

Metabolism is one that is relatively well-known. If you’ve ever heard that you shouldn’t eat dinner too late, the circadian system might be why. ”If we eat during the daytime, our metabolic system can respond to that food quickly. Whereas if we try to eat in the middle of the night, none of our systems are ready for that,” says Duffy.

Scientists are investigating how circadian rhythms affect other processes. Recently, researchers have been studying vaccine-fueled immune responses based on time of day and circadian rhythms, says Duffy.

These external cues are called zeitgebers, which is two German terms smashed together to mean “time giver.” Light, for example, is a big one. Because our eyes are connected to the SCN, light exposure can reset when these genes are activated or deactivated. Darkness can also impact melatonin production, aiding rhythmic sleep patterns by promoting the release of these hormones that make us drowsy. This can mean the circadian rhythms of visually impaired individuals may not be very synchronized to our day and night schedules, although melatonin and other supplements can help. Light is not the only factor to influence these rhythms–so can exercise, age, and temperature.

[Related: How to wake up when it’s still dark outside]

Our daily rhythms likely evolved because of the consistent light-dark pattern in the natural world, says Duffy. “If the first single-celled organisms were living in that strong environment of light and darkness, it would help them if they could anticipate it,” she says.