The Scientist: Michael J. Breus, Ph.D., a clinical psychologist and Fellow of the American Academy of Sleep Medicine
The Answer: If you got more sleep than you're used to last night, then you're probably tired because you got more sleep than you're used to last night.
Your body loves consistency. It prefers when you go to bed and wake up at the same time each day. If it's accustomed to getting eight hours of sleep and you give it nine, you're going to disrupt its rhythm. Mess with its rhythm and the body gets thrown off.
The average sleep cycle is about 90 minutes long, and individuals usually go through about five complete cycles over the course of the night (of course, it's not the same for everyone). That's 7.5 hours total. So if you snag an extra hour of sleep, you’re probably going to wake up in the throes of deep sleep, stages 3 and 4. Not only is it hardest to rouse yourself from the depths of stage 3 or stage 4 sleep, but doing so can leave you groggy and exhausted for the entire day. And remember that quantity does not equal quality. You can get a lot of low quality sleep and you'll still be sleep deprived.
If you want to figure out the best sleep schedule for you, try starting at the time you need to get up in the morning and count backward 7.5 hours. That ought to give you the five full sleep cycles your body craves.