You many think lying in bed unit 12 o'clock on the weekend is good from you, but you are wrong. Your biological clock tells you when you are tired. It also tells you when to wake up, and if you upset this by sleeping late on the weekend, it gets confused and you feel even more tired. A particular chemical is responsible for waking you up. It starts to work from 3 or 4 a.m. unit 11 a.m. If you sleep any later than this, you prevent the chemical from working, which in turn prevent you from feeling wide awake.