Are you a sleeper?

If you have short-term sleep deprivation such as skipping one or two nights of full sleep you can indeed make it up by sleeping longer later. However, long-term sleep deprivation such as sleeping 5-6 hours for weeks or months at a time is much harder to simply "catch up" on. If you are sleeping only 6 hours per night and try to catch up on weekends you are still functioning in a sleep deprived state that entire time.