"I'm fine on six hours"

Most people who stay up late have made a private calculation. Eight hours would be nice, and six is enough. They've been doing it for years, they hold down a job, and they haven't crashed the car. So the two hours taken from sleep look like a good deal, since they seem to cost nothing.

There's something in this. People do differ in how much sleep they need. The usual figure for adults is seven to nine hours, and that's an average with a range round it. One bad night does little harm, and the body copes with short runs of poor sleep better than the scare stories suggest.

The trouble is with the evidence you're using, which is how you feel. An experiment at the University of Pennsylvania tested that evidence directly. Forty-eight healthy adults lived in a laboratory. Some were allowed eight hours in bed a night for two weeks. Some got six hours, and some got four. Every day they did tests of attention and thinking, and rated how sleepy they felt.

The six-hour and four-hour groups got worse on every test, and they kept getting worse day after day for the whole fortnight. By the end, the six-hour group was performing about as badly as people who'd been kept awake for up to two nights running.

The sleepiness ratings are the part that matters here. They rose in the first few days and then flattened out. The people on six hours felt a bit sleepy and thought that was the extent of it. Their performance went on falling and their sense of being impaired didn't follow it. The researchers concluded that people were largely unaware of how far they'd slipped. They suggested this is why so many people believe cutting sleep is harmless.

That's the difficulty with "I'm fine". The judgement is being made by the instrument that's affected. You've also been on short sleep for so long that you've nothing recent to compare with. What feels like your normal self may be your six-hour self, and you may not have met the other one since your last long holiday.

The costs aren't only about concentration. A large review pooled 154 experiments in which people's sleep was cut short or broken up, covering more than 5,700 participants. Across all kinds of sleep loss, people felt less positive emotion and more anxiety. Notice what that does to this book's subject. After a short night, the day is greyer and more worrying than it would have been. That's the day you'll want to take revenge on in the evening.

I should be fair about the limits. The Pennsylvania experiment had 48 people, all healthy and aged between 21 and 38, in a laboratory. Six hours was their time in bed, so they slept a little less than that. You aren't in a laboratory, and you may need less sleep than they did. There are said to be a few people who do well on six hours for life. I don't know if you're one. There's a simple test, though, and it's in the title of this book. People who need only six hours aren't tired at ten. If your eyes are closing on the sofa, your body has told you what it needs tonight, whatever the average is.

One more check is available. Look at what you do with alarm-free mornings. If you sleep two or three hours longer every Saturday, the six hours wasn't enough.