A study in 2017 found that “social jet lag” creates more issues than you might think, according to¬†Bustle.

The habit of getting up later and going to bed in the early a.m. on the weekends is widespread, but the researchers behind the study, from the American Academy of Sleep Medicine, found that people who tend to maintain this kind of schedule suffer from lower mood, more health issues and more sleep dysfunction than people who maintain a kind of sleep schedule throughout all seven days of the week. Weekends are a human invention (we all remember Maggie Smith on Downton Abbey’s incredulity when they were introduced to Britain in the early 20th century), and their disruption isn’t built into human biology.

Dr. Mohammad Amin from the Stony Brook Medicine Sleep Disorders Centertells Bustle that it has to do with the body’s circadian rhythms. Everybody has a complex system of internal “clocks” that tell you when to wake up and when to sleep, involving hormones, light sensitivity, parts of the brain, and other mechanisms. We’re still understanding how circadian rhythms work, but they’re what goes haywire in your body when you experience jet lag or severe sleep deprivation.