Sleep Calculator

A night isn't one long block of sleep. You move through repeating cycles of roughly 90 minutes — light sleep, then deep sleep, then REM — and each one has a different job.

Deep sleep is front-loaded. Most of it happens in the first two or three cycles. It's the physically restorative part: tissue repair, immune function, the reason you feel wrecked after a genuinely short night.

REM is back-loaded. The proportion of REM grows through the night, and the longest stretch comes in the final cycles. REM is heavily involved in memory consolidation and emotional processing.

This is the part most people miss: cutting your night from six cycles to four doesn't remove a third of your sleep evenly. It removes most of your REM, because you delete the end of the night rather than a slice of all of it. That's why a short night leaves you emotionally thin rather than just physically tired.

Being pulled out of deep sleep produces sleep inertia — that heavy, disoriented fog that can last twenty or thirty minutes. Waking at the end of a cycle, when you're already in light sleep, avoids most of it.

It's why six hours can occasionally leave you sharper than seven. You didn't sleep better; you woke at a kinder moment.

Be sceptical of precision here, including ours. Cycles vary between roughly 70 and 120 minutes, they lengthen through the night, and they differ between people and between nights. The 90-minute figure is an average, not a schedule.

The instinct is to repay the debt in full — sleep five hours tonight, so sleep eleven tomorrow. That is the wrong move, and it's how one bad night becomes a bad week.

Keep your wake time. This is the rule that matters. Your body clock is anchored by when you get up and see light, not by when you go to bed. Sleeping until noon on Sunday shifts the clock later and makes Sunday night harder, which makes Monday worse.

Go to bed earlier instead. Move bedtime forward by 30 to 60 minutes for two or three nights. That's roughly one extra cycle, and it's how recovery actually happens.

• A nap helps, with rules. Either 20 minutes, or a full 90 minutes. Thirty to sixty minutes lands you in deep sleep and you'll wake worse than you started.
• Nap before 3pm. Later than that and you're borrowing from tonight.
• Get outside in the morning, especially after a bad night. Light is what re-anchors the clock, and it does more than the coffee will.

Two or three nights of decent sleep restore most function. What doesn't work is chronic short sleep during the week and a lie-in at the weekend — that pattern keeps your clock permanently unsettled, which is why Monday feels like jet lag. There's more on this on our Circadian Rhythm page.

Most healthy adults land between five and six cycles — roughly seven and a half to nine hours. A small minority genuinely function on less, and a similar minority need more.

The honest test isn't a number. It's these three questions:

• Do you wake before your alarm, at least sometimes?
• Do you get through mid-afternoon without fighting your eyelids?
• On a free morning, do you sleep dramatically longer than usual? If yes, you're running a deficit during the week.

If you're consistently below five cycles and feel fine, be a little suspicious of that feeling — tired people are unreliable judges of how tired they are. And if you sleep a full night and still wake unrefreshed, that's a conversation for a doctor, not a calculator.