What time should you go to bed?

How this calculator works

A night's sleep isn't one continuous block. You move through light sleep, deep sleep and REM in cycles that average about 90 minutes, repeating four to six times a night.

Waking at the end of a cycle — when sleep is lightest — generally feels easier than being woken mid-cycle out of deep sleep. That heavy, foggy feeling when an alarm catches you at the wrong moment is called sleep inertia, and it can take a while to shake off.

So the calculator counts backwards (or forwards) in 90-minute blocks, and adds the time it takes you to actually drop off — 15 minutes by default, which you can adjust above.

How many cycles do you need?

• 6 cycles (~9 hours) — a full night for most adults.
• 5 cycles (~7.5 hours) — the realistic target for most people most nights.
• 4 cycles (~6 hours) — workable occasionally, not as a habit.
• 3 cycles (~4.5 hours) — short. Fine once; a problem if it repeats.

Adults generally need seven to nine hours a night. Cycle length varies between people and even across a single night — early cycles carry more deep sleep, later ones more REM — so treat 90 minutes as a useful average, not a stopwatch.

Getting more out of it

• Keep the wake time steady. A consistent rising time anchors your body clock more effectively than a consistent bedtime.
• Get daylight early. Light in the morning is the strongest signal your circadian rhythm responds to.
• Watch caffeine's tail. It has a half-life of around five hours, so an afternoon coffee is still working at bedtime.
• If you're not asleep in ~20 minutes, get up. Do something dull in low light and return when you feel sleepy.

Common questions

Is 90 minutes exact?

No. It's an average. Real cycles range roughly 70–120 minutes and change through the night, so use these times as a target to aim near, not hit precisely.

Should I pick the latest bedtime that still works?

Not usually. Consistently choosing the shortest option builds sleep debt. Aim for five or six cycles on most nights.

I still wake up tired after a full night. Why?

Duration is only part of it — fragmented sleep, timing that fights your body clock, alcohol, and untreated conditions all affect how restorative sleep is. If it persists, it's worth speaking to a healthcare professional.

Find out what's wrecking your sleep →