Sleep calculator

How the sleep calculator works

Sleep runs in cycles. The US National Heart, Lung, and Blood Institute (NHLBI) says a cycle starts over every 80 to 100 minutes, and that most nights have four to six of them. The calculator takes 90 minutes, the middle of that range, as one cycle.

• If you know when you need to wake up, it counts back 6, 5 and 4 cycles (9, 7 and a half, and 6 hours) and then takes off the time you usually need to fall asleep. That gives you three bedtimes.
• If you know when you are going to bed, it counts forward the same way and gives you three wake-up times. Press "use the time now" if you are going to bed straight away.

The time to fall asleep is set to 15 minutes. Change it to 5 or 30 if that is closer to how long you usually lie there.

For example, to wake at 7am with 15 minutes to drop off, the bedtimes are 9:45pm (6 cycles), 11:15pm (5 cycles) and 12:45am (4 cycles).

What happens in a sleep cycle

Each cycle moves through three stages of non-REM sleep and then a stretch of REM sleep, the stage when most dreaming happens. Stage 3 is deep sleep, also called slow-wave sleep. The NHLBI says you usually spend more time in deep sleep early in the night, and have more REM sleep later on. So the cycles are not identical: those early in the night hold more deep sleep, and later ones more REM.

Why wake at the end of a cycle?

The groggy, slow feeling just after you wake is called sleep inertia. A 2019 review of the research found it is worse when you wake from deep sleep. In one of the studies it describes, people woken from slow-wave sleep after a nap performed 41% worse than they had before the nap, while people woken from lighter stage 2 sleep performed about the same as people who had stayed awake.

That is the idea behind timing your alarm to the end of a cycle. It is a rough guide rather than a precise one, because your cycles vary in length from night to night, and you can't know exactly when you fell asleep.

How much sleep do I need?

The NHS says adults need 7 to 9 hours a night and children 9 to 13 hours. Five or six cycles fall in the adult range. Four cycles is 6 hours, which is less than most adults need, so the calculator flags it.

If one of the cycle-based bedtimes means less sleep than you need, pick the earlier one.

If you can't sleep

The NHS advice for poor sleep includes going to bed only when you're sleepy, waking up and getting out of bed at the same time every day, and not sleeping in after a bad night but sticking to your regular hours. A fixed wake-up time is the easiest place to start, which is why the calculator begins with it.

See a GP if changing your sleep habits hasn't helped, you've had trouble sleeping for months, or it is affecting your daily life in a way that makes it hard to cope.

For what the research says about magnesium, read magnesium and sleep. If you're thinking about a sleep supplement, melatonin in the UK explains where it stands here, and sleep supplements in the UK covers the rest. If you drink coffee or tea in the afternoon, the caffeine calculator adds up how much caffeine you have in a day.