Sleep pressure

If you've ever dozed off on the sofa in the evening and then lain in bed an hour later feeling oddly awake, or fought to keep your eyes open in the afternoon after a short night, you've felt it in action. Both come down to how much of the drive to sleep you're carrying at that moment.

This drive is what's commonly referred to as sleep pressure. It's one half of how sleep is timed, and once you know how it behaves, a lot of everyday sleep makes more sense.

Two systems deciding when you sleep

One of the main frameworks in sleep research is the two-process model, first set out in the early 1980s and refined since (Borbély et al., 2016). It says two systems work alongside each other.

The first is sleep pressure, which the model calls Process S (the homeostatic process, meaning the part that keeps a balance). It builds steadily the longer you're awake and falls away while you sleep. The second is the body clock, Process C (the circadian process), which sets the times of day when sleep comes easily and when it's hard to come by. You fall asleep most easily when the pressure is high and the clock is signalling night at the same time.

What builds up while you're awake

One of the substances that carries this pressure is adenosine, a by-product of the brain's energy use. As the brain works through the day, adenosine builds up in the spaces between nerve cells and damps down the cells that keep you alert. Sleep gives it time to clear.

The clearest direct evidence comes from animal work. Porkka-Heiskanen and colleagues (1997) measured adenosine in a wake-promoting region at the base of the brain in cats. It rose during spontaneous waking, kept rising the longer the animals were kept awake, and fell slowly during the sleep that followed. Caffeine works by blocking the receptors adenosine acts on, which is why a coffee makes you feel less sleepy without removing the pressure underneath. The cat work is the most direct measurement we have; in people the evidence is less direct, so we treat adenosine as one carrier of sleep pressure rather than the whole of it.

When a short night leaves pressure behind

If the night is too short to clear the pressure, some of it carries into the next day. Van Dongen and colleagues (2003) tested this in a laboratory with 48 healthy adults aged 21 to 38. Some were limited to 4, 6 or 8 hours in bed a night for two weeks; others stayed awake for three nights. Six hours or less a night built up deficits in attention and thinking that grew day by day, and by the end of the fortnight they matched up to two nights without any sleep. Yet people's own ratings of sleepiness rose at first and then barely climbed further, so they were largely unaware of how much their performance had slipped. For that reason we don't rely on how sleepy you feel to judge whether you're getting enough sleep.

Naps and evening dozes

The pressure can also be spent at the wrong time. A nap or an evening doze clears some of it hours before bed, so there's less left to carry you into sleep at lights out.

Werth and colleagues (1996) showed this in healthy young men who took an early-evening nap in the laboratory. That night they took longer to fall asleep, and the brainwave pattern of deep sleep in the first part of the night was weaker than usual. The researchers could reproduce the pattern closely with the two-process model, which is what we'd expect if some of the pressure had already been used. It was a laboratory study in young men and can't tell us how big the effect would be for any one person at home.

Using this in your plan

Your sleep diary records when you sleep, how long it takes you to drop off, and any naps or dozes. If your nights are short, the plan may ask you to give sleep a little more time, moving lights out earlier in small steps and watching in the diary whether the extra time turns into sleep. If you're dozing in the evenings, the plan will help you stay up and alert until bedtime so the pressure is still there when you need it.

Dozing off often without meaning to, even after a full night in bed, isn't something to manage with habits alone. Mention it to your GP, because it can be a sign of a sleep condition that needs looking at properly.