Did you go to bed last night, fall asleep quickly, and wake this morning refreshed, full of energy, and raring to go? If you did, you are either young or lucky. One of the most common complaints that doctors hear from patients, particularly older patients, is of difficulty in obtaining a good night's sleep.

Why do we need sleep at all? What makes us fall asleep? And once we are asleep, what makes us wake up?

Good questions, to which science is only recently providing answers. Most of us spend roughly a third of our lives asleep, but we know remarkably little about something that we do so much. The current understanding of sleeping and waking runs roughly as follows:

There is a sleep-debt meter in your brain. As soon as you wake up in the morning, that meter turns on. It keeps running as long as you remain awake. The sleep-debt meter is remorseless, and if you short-change yourself with sleep one night you will pay with increased drowsiness the next day. The only way to turn the meter back is to go to sleep, at which point your sleep debt will be discharged at about two hours for every hour you sleep: sixteen hours awake, eight hours asleep.

Clearly, that can't be the whole story. If it were, you would gradually feel sleepier and sleepier as the day wears on. You don't, because another part of the brain generates wake-up calls. It sends a rather weak signal in the morning, when normally you don't need one because you have slept all night and the tally on your sleep-debt meter is low. Your wake-up caller sends an even weaker signal in the afternoon, which is why many people feel the urge for an afternoon nap. And the wake-up call becomes very strong in the early evening hours, when it has to combat an increasing load of sleep debt. Late in the evening, the wake-up calls turn off, the sleep debt is large, and we know it's bedtime.

Your sleep-debt meter keeps running, regardless of time and place; however, your wake-up caller has a 24-hour cycle that is strongly tied to your normal day/night routine. Usually these two brain activities balance nicely, giving us our nights asleep and days awake. But we have found ways to mess with that rhythm.

Suppose you fly from East Coast to West Coast. You go to bed at eleven West Coast time, which is two in the morning by your internal clock. Your sleep-debt is large, so you ought to sleep at least eight solid hours. Instead, you wake up at four. You know that you are tired and you feel terrible, but there's nothing you can do about it. It's that pesky wake-up section of your brain, still locked in to its East Coast daily cycle.

We call it jet lag, but that puts the blame in the wrong place. Brain-lag would be a better name.

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