Sleep debt

The brain keeps track of the accumulated hours of wakefulness.  One hour of sleep is needed to "pay off" every two hours of wakefulness.  When the sleep debt load is high enough, the brain will essentially force one into sleep, which is what happens when drowsy drivers fall asleep at the wheel.  Although sleep debt must be paid back eventually, there is unfortunately no way to "stockpile" sleep in the event of future shortages.  Dement notes, too, that there have been no studies on long term sleep debt, and so it is unknown whether the brain keeps track of sleep debt past two weeks or so.  And while a large sleep debt can be a very dangerous thing, a small sleep debt is good, because it allows us to fall asleep quickly and stay asleep.  Test subjects with no sleep debt often have difficulty both falling asleep quickly and staying asleep the whole night through.

Source:

Dement, The Promise of Sleep