What happens when you don't get enough sleep? Strangely, it's much like overspending; you accumulate a debt. Suppose you manage to get only five hours of sleep on a particular night, and you need eight. The following day you will have a sleep debt of three hours -- and you will feel it. You will likely feel lousy, and likely drink several cups of coffee to stay awake. Like the bank, your body doesn't forget: when you owe it a debt, it wants to be paid back, and it won't let you feel good until you settle your account.
A sleep debt can arise in several ways. You can, for example, get an hour less sleep than you need over the five work days of the week. You then have a debt of five hours. This is why many people sleep in on the weekend -- they are trying to pay it off. Fortunately there is something about a sleep debt that is unlike a monetary debt. You can accumulate a relatively large debt, say ten hours, over a few days, but you don't have to pay back all ten hours to feel better. Usually after one night of a ten hour sleep you'll feel much better. Nevertheless, you have to be careful of a sleep debt, particularly if you are driving.
Surprisingly, a sleep debt can also help you. When you are getting ready for bed at night you have a large sleep debt from the 16 hours you have been awake. Since you are awake two hour for every hour you sleep, your debt is 8 hours. But with a debt of 8 hours you will not fall asleep very fast, and you might wake up in the night. If you have a slightly larger sleep debt, say 10 hours, you will fall asleep fast and stay asleep. Of course we don't want too much of a sleep debt so we have to find a happy balance.
