"I'm not tired any more"

Something odd often happens in the late stretch. At ten you could barely keep your eyes open. By half past eleven you feel reasonably awake. It seems as if the tiredness was a passing dip, and that going to bed at ten would have been a mistake, because you've clearly got more in you.

I don't doubt the feeling. The question is what produces it. I'd suggest two things, and neither is a sign that you've had enough rest.

The first is what you're looking at. A research team in Boston had people read in the hours before bed. On some evenings they read on a device that gives off light, and on others they read a printed book. The same people did both. On the screen evenings, people felt less sleepy at bedtime. They produced less melatonin, which is the hormone that signals night to the body. They took longer to fall asleep, and their body clocks shifted later. The next morning they were less alert. They were reading both times, so the difference points to the light.

Think about what that means for the second wind. It appeared while you were looking at a bright screen, and by the study's measures it left you less alert the next morning. It was a laboratory study, and the size of the effect in ordinary homes is still argued over. The direction is what matters here. The light was covering the tiredness up, and the tiredness was still there when the alarm went.

The second is the material. A good drama is built to hold your attention, and the episode ends on a hook to keep you there. A phone feed offers something new every few seconds. An interested mind is an alert mind. Again, that's your sleepiness being overridden for a while. It will be back at seven in the morning.

There are two other common causes, and they belong to the loop from chapter 2. One is the doze. You nod off in front of the television at nine, wake at half past with a stiff neck, and find you're no longer sleepy. The standard account of sleep says that pressure to sleep builds through the waking day and is released by sleeping. Even a short sleep lets some of it go. So the doze hasn't shown that you weren't tired. It has spent some of your tiredness early, on a sofa, in a position that did your neck no good. If you catch yourself going under at nine, that's the tired point arriving early, and the useful response is to go up.

The other is caffeine. A tired day runs on coffee, and the last cup tends to creep later into the afternoon. One study gave people a large dose of caffeine, about what several cups of coffee contain. They took it at bedtime, three hours before, or six hours before, and their sleep was measured at home. All three timings disturbed sleep compared with a dummy pill, including the dose taken six hours ahead. So a strong coffee at four can still be at work at ten. The short night leads to the late coffee, and the late coffee helps produce the second wind. If you drink coffee after lunch, it's worth a fortnight's experiment to stop at noon and see whether ten o'clock feels different.

So I'd treat the two readings differently. At ten, in a quiet moment, your body reported that it was tired. At half past eleven, under bright light and halfway through a plot, it reported that it wasn't. I'd trust the ten o'clock reading.

There's a related finding about what happens upstairs. Two researchers surveyed 338 young adults and separated two times that usually get treated as one. The first was when people went to bed, and the second was when they tried to go to sleep. The average gap was 39 minutes, much of it spent on screens. People with a gap of more than an hour were about nine times as likely to be poor sleepers as people who went to sleep straight away. So for many people, going to bed only moves the late stretch upstairs. When I say bedtime in this book, I mean lights out.

Now the honest exception. Some people really aren't sleepy at ten, or at eleven either. Body clocks differ, and a minority of people are built to sleep from one till nine. A German diary study of 108 employees found that these evening types reported more delayed bedtimes on work nights. The pattern was what body-clock science would predict, and it wasn't explained by poor self-control. If that's you, your problem isn't the one in this book. Your sleep is set to a different timetable from your job. This book's title gives the test once more. It's about people who were tired at ten. Chapter 12 comes back to night people.