Sleep Calculator: What Time Should You Go to Bed?

How this calculator works

Sleep isn't one flat state you drop into and climb out of. It runs in cycles — light sleep, deep sleep, then REM — and each full lap takes roughly 90 minutes. You go through four to six of them a night.

Where in that lap your alarm catches you matters more than most people realise. Ripped out of deep sleep, you get sleep inertia: the heavy, disoriented fog that can last 15 to 60 minutes and makes the snooze button feel like the only reasonable option. Woken at the end of a cycle, when you're already close to the surface, the same amount of sleep feels dramatically better.

So the calculator works backwards from your alarm in 90-minute steps, and adds the time it takes you to actually fall asleep. That second part is the piece most sleep calculators quietly skip — and it's the reason their answers are consistently 15 minutes optimistic. A bedtime is not a sleep-onset time. If you're in bed at 23:00 and asleep by 23:15, the cycle clock starts at 23:15.

What a sleep cycle actually contains

A single cycle moves through three broad stages:

• Light sleep (N1–N2) — the entry and exit ramp, and the bulk of the night. Easy to wake from, which is exactly why you want your alarm here.
• Deep sleep (N3) — slow-wave sleep, concentrated in the first half of the night. This is the physically restorative part, and the one that makes an alarm feel violent.
• REM — dreaming sleep, tied to memory and emotional processing. REM periods get longer with each cycle, so the last stretch before your alarm is REM-heavy.

Because deep sleep front-loads and REM back-loads, cutting a night short doesn't shave a bit off everything evenly — it disproportionately cuts REM. That's why five hours feels worse than the arithmetic suggests.

Where the 90-minute rule breaks down

Ninety minutes is an average, not a law. Real cycles run anywhere from about 70 to 120 minutes, they vary between people, and they vary within one night — your first cycle is typically shorter than your last. Sleep debt, alcohol, late caffeine, illness, stress and an irregular schedule all shift the picture.

So treat the times below as good starting targets, not precision instruments. If a suggested bedtime consistently leaves you groggy, your cycle probably isn't 90 minutes. Try shifting your bedtime 15–20 minutes earlier or later for a week and see which direction improves the morning. That's a more reliable signal than any formula.

How much sleep do you actually need?

For most adults the answer is seven to nine hours, which is five to six cycles. Four cycles (six hours) is survivable for a night; three (four and a half) is an emergency measure, not a plan. Teenagers need more — eight to ten hours — and consistently get less than almost any other group.

One caveat worth stating plainly: hitting a cycle boundary on five hours of sleep does not make five hours enough. Cycle timing changes how the wake-up feels; it doesn't replace the sleep you didn't get. If you're choosing between "wake at a cycle boundary after 6 hours" and "wake mid-cycle after 7.5 hours", take the 7.5.

How to find your own cycle length

The only way to move past the average is to measure. Movement during the night follows your sleep stages closely enough to map them: you're near-motionless in deep sleep and shift position around cycle boundaries. Track a couple of weeks of nights, look at where your quiet blocks end, and you'll see your own rhythm — including how long you really take to fall asleep, which is usually longer than people estimate.

Frequently asked questions

What time should I go to bed to wake up at 6 a.m.?

Allowing 15 minutes to fall asleep: 20:45 for six cycles (9 hours), 22:15 for five cycles (7.5 hours), or 23:45 for four cycles (6 hours). Five to six cycles is the range to aim for on a normal night.

Is it better to sleep 6 hours or 7.5 hours?

7.5 hours, in nearly every case. Six hours is four complete cycles and does land on a boundary, but it sits below what most adults need, and the deficit accumulates night after night as sleep debt. Cycle timing improves how a wake-up feels; it doesn't create sleep you didn't have.

Why do I feel worse after a long sleep than a short one?

Usually because the long one ended mid-cycle. Waking during deep sleep produces sleep inertia regardless of how much you slept beforehand — nine hours interrupted at the wrong moment can genuinely feel worse than seven and a half timed well.

Does the 90-minute rule work for naps?

Two nap lengths work, for opposite reasons. Around 20 minutes keeps you in light sleep and skips inertia entirely. Around 90 minutes completes one full cycle and returns you to light sleep before you wake. The 45-to-60-minute middle is the worst option — it drops you into deep sleep and pulls you out of it.

Does this calculator account for waking during the night?

No — it assumes an uninterrupted night, which nobody has exactly. Brief awakenings between cycles are normal and mostly unremembered. If you're waking fully and repeatedly, that's worth tracking, because it changes the arithmetic far more than a 15-minute bedtime adjustment does.