The sleep jar is a concept that we bandied about at University, but I think it was Jamz that came up with it.
The concept is quite simple:
When you sleep, you add to the sleep jar
When you are awake, you are taking from the sleep jar
So, if you are awake too long, your sleep jar gets too empty, to the point where you actually owe the sleep jar.
So, if you think about it, if you are in the middle of a all-nighter, you can grab a quick 4 hour nap, but if you carry on for too long, you have no choice but to get a solid 8-hour sleep.
However, even if you get a good 8 hour sleep after 2 days awake, you probably still aren't back to normal. I know personally that I need 2 consecutive nights of 10 hours after an all-nighter.
Further, if you are trimming an hour off every night's sleep, then your sleep jar balance is going to gradually deplete, and after a few weeks, you will end up owing the sleep jar.
When you owe the sleep jar, I think the body is more susceptible to illness.
I think that's what happened to me last week: I owed the sleep jar and got the flu.
I think since Wednesday last week, I have slept about 10 hours a day. I am on the upswing now, and feel great.