Sleep calculator: what time to go to bed

How sleep cycles work

Sleep is not a straight line. Over a night you go through a cycle four to six times, each lasting on average about 90 minutes: light sleep, deep sleep, then REM sleep (the stage in which you dream). At the end of each cycle there is a short window of very shallow sleep. That is when you wake up most easily.

This is where the odd feeling comes from that eight hours can leave you more tired than seven and a half. It is not just how long you sleep that matters, but where the alarm catches you. If it goes off in the middle of deep sleep, your body needs tens of minutes to come out of it. The state is called sleep inertia, and it is the reason the first two coffees don't seem to help.

Why we added the time it takes to fall asleep

Almost every sleep calculator online simply subtracts eight hours from your wake-up time. The problem is that nobody falls asleep the second they switch off the light. An adult needs 10 to 20 minutes on average; this is called sleep latency. If you consistently fall asleep in under five minutes, it usually doesn't mean you sleep well, it means you are sleep deprived.

How many hours you actually need

The recommendation for adults is between 7 and 9 hours, which means five or six complete cycles.

• 6 cycles, 9 hours. The option for days with heavy physical effort, illness or after a week of short sleep.
• 5 cycles, 7 and a half hours. The target for most adults, on most days.
• 4 cycles, 6 hours. An emergency fix for one night, not a plan for a month.

Regularity matters more than the exact time

Your internal clock doesn't look at the clock on the wall. It calibrates itself by light and by repetition. A bedtime you roughly stick to, weekends included, does more than the “perfect” time you hit once every three days. A gap of more than an hour between your weekend schedule and your Monday schedule even has a name: social jet lag.

Light remains the strongest signal. Fifteen minutes of natural light in the first hour after waking anchors your rhythm better than any supplement, while bright screens in the hour before bed push it in the opposite direction.

Where melatonin fits in

Melatonin is not a sleeping pill and it doesn't “knock you out”. It is the hormone your body produces in the evening anyway, and its role is to signal that night has come. The authorised claim in the European Union states that melatonin contributes to the reduction of time taken to fall asleep, and the beneficial effect is obtained by consuming 1 mg of melatonin close to bedtime.

Timing matters more than the dose: take it 30 to 60 minutes before the time you want to sleep. In practice, that is the time the calculator tells you to start your evening routine, not when you are already in bed with your phone in your hand.

What this calculator doesn't do

The 90-minute cycle is an average. For some people it lasts 80 minutes, for others 110, and for the same person it varies from one cycle to the next over the night: the first cycles have more deep sleep, the last ones more REM. Use the times as a guide, not as a rule.

The calculator doesn't take into account age, shift work, sleep apnoea or treatments that affect sleep. If you consistently sleep badly despite a regular schedule, that is a conversation to have with a doctor, not with a calculator.