you may think lying in bed until 12 o'clock in the weekend is good for you but you are wrong your biological clock tells you when you are tired it also tell you when to wake up,and if you upset this by sleeping late on the weekend,it gets confused and feel even mor tired. a particular chemical is responsible for waking you up. it starts to work from 3 or 4 until 11 a.m. if you sleep any later than this you prevents you from feeling wide awake