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

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. Nature only barters, not lends.