Why do I feel less tired if I only get a small amount of hours of sleep at night as opposed to a more medium amount (but still less than what I "need")? The other night, I only slept for 3 hours because I was up late talking to a friend and I didn't feel tired **at all** the next day in class. Normally, I get 6 hours of sleep (I know I need 8 - 9 as a high school student) and I feel completely exhausted. Why is that? 