Let's say because of a darn lab report or assignment, you miss 1 hour of sleeping time. You try to compensate the loss by sleeping through your 1 hour bus journey, but you still feel tired. Then you sleep through your 2 hours lecture (not uncommon though) and you still feel tired. Then you sleep again on your way home. Still tired. You can't take it anymore and decide to turn in early after dinner.
On paper, you are clearly getting way more sleep than you normally do, but in reality, oh well......
At least don't need to count sheep anymore.