The main culprit is what's known as social jet lag. Like traveling to a different time zone, staying out late (or, let's be real, watching Stranger Things until 3am) and then sleeping in late the next morning affects your body's internal clock, or circadian rhythm, says Michael Breus, PhD, a board-certified sleep specialist and fellow of the American Academy of Sleep Medicine. "If you do this two days in a row, you end up with Sunday night insomnia."
Another factor effing up your sleep schedule is, not surprisingly, alcohol. It causes you to wake up more frequently during the night and suppresses the release of the antidiuretic hormone ADH, which makes you go to the bathroom more often, Dasgupta says. This means your sleep quality after a night of drinking is pretty poor, and poor sleep quality means you wake up even later because you're not close to feeling refreshed. And so the vicious (lack of) sleep cycle continues.
Finally, there's stress. "Whether you're a student worrying about tests, a parent taking care of your kids, or working at a 9-to-5, stress is a factor for pretty much anyone," Dasgupta says. When your body is stressed out -- physically, mentally, or emotionally -- it releases the hormone cortisol, which causes your mind to go on high alert and disrupts sleep. It's a chicken-and-egg situation: if you're stressed, you sleep poorly; but poor sleep can also cause you to feel stressed. Either way, this delightful combo can lead to a host of health risks, Dasgupta says, like high blood pressure and diabetes -- not to mention a general shitty mood.