Other shocking results of the study included the fact that staying up and sleeping in late made people more cranky and tired. This revelation is the actual reason you shouldn't sleep in too much on the weekends. Your body gets used to waking up at a certain time each day, and for most adults that's the time that you have to get up for work. When you sleep in two days out of seven, you disrupt the rhythm that your body has already established. If you just sucked it up and got out of bed at the same time over the weekend, you'd cement your schedule. Eventually you'd wake up at that time naturally, which means you'd be less tired overall, and come Monday morning you wouldn't feel exhausted because you wouldn't be trying to shift your circadian rhythm back. The key is consistency. Right now it sounds like a nightmare to try to get up at 7 a.m. on a Saturday. But if you just did it for a few weeks, you'd get used to it.