"It's only a couple of hours"

It's only a couple of hours in the same way that a flight from London to Athens is only a couple of hours. Your body clock doesn't measure the number. It measures the timing.

The mechanism is this. You have an internal clock, and it runs on a cycle a little longer than a day, and it's held to twenty-four hours mainly by light, by when you see it and when you don't. Sleep late and you see morning light two or three hours later than usual, and you're awake and lit up later into the evening, and the clock takes that as a signal to run later. It's a small nudge, but it's a nudge in the same direction two nights running.

Yang, Spielman and colleagues tested exactly the pattern in this book. Ten healthy young adults had their sleep times pushed two hours later on Friday and Saturday night, to copy an ordinary weekend, and the researchers measured the timing of their melatonin rise, which is the body's own marker of when night begins. After the weekend it came about thirty-two minutes later. That's from two hours of lie-in for two nights. On Sunday night the participants took longer to fall asleep, and on Monday morning they were sleepier. A single dose of melatonin on Sunday afternoon reversed it, which is interesting and not something I'd suggest, but which shows the mechanism is the clock and nothing else.

Stephanie Crowley and Mary Carskadon did the same with older teenagers, who go to bed an hour and a half later and get up three hours later on a typical weekend, and the clock moved by about forty-five minutes over two nights. Teenagers' clocks run later anyway, so the effect may be bigger in them, but the direction is the same.

So take half an hour to three quarters of an hour as the size of the drift from a normal weekend, in a study, with the numbers as they are. Now put yourself in bed on Sunday night at half past ten. Your clock thinks it's ten, or a quarter to. You're being asked to sleep before your own night has started, and that's why you're staring at the ceiling, and that's why Monday's alarm at half past six arrives at what your body counts as six, or a quarter to. The tiredness isn't the two hours you didn't sleep on Sunday. It's the thirty or forty minutes you're now out of step, on top of a short night, and it takes a couple of days of getting up early to pull the clock back, which is why Wednesday is the first normal day.