you may think that lying in bed until 12 o'clock on the weekend is good for you, but you are wrong. Your biological clock tells you when you are tried. 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 tried. A particular chemical is responsible for waking you up. It starts to work from 3 a.m. If you sleep any later than this , you prevent the chemical from working , which in turn prevent you from feeling wide awake.