Speaking about sleep in terms of debt is somewhat misleading. A sleep deficit can't be "paid off" as you might pay off a financial obligation. If you overdraw from a checking account, you can settle up by making one larger-than-usual deposit. If you stay awake for three days, then sleep much longer than usual doesn't give you a full cognitive recovery. It might be more accurate to think of staying awake as taking out a loan with very high interest.
EDIT: This is a difficult concept to understand because it sounds like a semantic difference. Here's an interesting article that explains it in more detail:
Well, not really. During the time where you're building up those 50 'lost hours', your body is performing worse. What you owe in sleep you'll (immediately) pay for in reduced quality of wakefulness: lower mental acuity, worse immune system, poorer physical performance, slower recuperation, etc.
EDIT: This is a difficult concept to understand because it sounds like a semantic difference. Here's an interesting article that explains it in more detail:
http://www.npr.org/2013/08/15/212276021/of-neurons-and-memor...