← Back to blog

Sleep calculator

Quick answer

What time should I go to bed?

Count back from your wake-up time in 90-minute cycles and add about 15 minutes to fall asleep. For a 6:00 AM alarm that means 10:15 PM for five cycles, or 8:45 PM for six. Pick your own time below.

To wake up at 6:00 AM on a cycle boundary, fall asleep at one of these:

  • 8:45 PM6 cycles · 9 hours
  • 10:15 PM5 cycles · 7 hours 30 min
  • 11:45 PM4 cycles · 6 hours
  • 1:15 AM3 cycles · 4 hours 30 min

Assumes about 15 minutes to fall asleep and 90-minute cycles. Both vary from person to person and night to night — treat these as targets, not deadlines.

Open the full page for 6:00 AM →

If you need to wake up at…

Pick your alarm time to see every bedtime that lands on a cycle boundary.

4:00 AM4:15 AM4:30 AM4:45 AM5:00 AM5:15 AM5:30 AM5:45 AM6:00 AM6:15 AM6:30 AM6:45 AM7:00 AM7:15 AM7:30 AM7:45 AM8:00 AM8:15 AM8:30 AM8:45 AM9:00 AM

If you are going to bed at…

Pick when you are actually turning the light off, and see when to set the alarm.

9:00 PM9:15 PM9:30 PM9:45 PM10:00 PM10:15 PM10:30 PM10:45 PM11:00 PM11:15 PM11:30 PM11:45 PM12:00 AM12:15 AM12:30 AM12:45 AM1:00 AM1:15 AM1:30 AM

How the maths works

A night of sleep is not one continuous state. You move through light sleep, deep sleep and REM in cycles that average about 90 minutes. Waking at the end of a cycle, in light sleep, feels dramatically easier than being pulled out of deep sleep halfway through one — that horrible hit-by-a-truck feeling has a name, sleep inertia.

So the calculator counts whole cycles from the time you give it and adds about 15 minutes for falling asleep, because you do not lose consciousness the second your head lands.

Where these calculators oversell themselves

Almost every sleep calculator online presents 90 minutes as a fixed constant. It is not. Cycles run roughly 80 to 110 minutes, they get longer towards morning, and they differ between people and between nights. Nobody can predict your boundaries to the minute without measuring your sleep.

What that means in practice: these times are good targets, and a genuinely useful way to choose between two bedtimes. They are not worth losing sleep over — literally. If landing on a boundary would cost you an hour of total sleep, take the sleep. Sleep-cycle alarm clocks have the same limitation.

The part the calculator cannot solve

Choosing the right time is the easy half. The hard half is being conscious when it arrives — and if you routinely dismiss alarms without remembering it, no amount of cycle maths will help. That failure happens on autopilot, before your rational brain is online, which is why the only reliable fix is an alarm you have to earn your way out of.

Frequently asked questions

How does a sleep calculator work?

It counts backwards or forwards in whole sleep cycles of about 90 minutes, adding roughly 15 minutes for the time it takes to fall asleep. The aim is to have your alarm land at the end of a cycle, in light sleep, rather than in the middle of deep sleep.

How accurate is the 90-minute sleep cycle?

It is an average rather than a rule. Cycles run roughly 80 to 110 minutes, lengthen through the night, and vary between people. The calculator gives you sensible targets, not a precise schedule.

Should I sleep less to wake up on a cycle boundary?

No. Total sleep matters more than cycle timing for most adults, who need seven to nine hours. Use the boundary idea to choose between bedtimes you were already weighing up, never as a reason to cut sleep short.

What if I set the perfect time and still sleep through it?

That is a different problem and a very common one. In the first minutes after an alarm your rational brain is still offline, so dismissing it is a reflex you can perform while effectively asleep. The fix is an alarm that needs a real task completed before it stops.

Why people use Captain Wake to wake up on time

Not because it rings louder. Because it removes every way your half-asleep brain talks you back into bed.

There is no button to tap

Swiping an alarm away is a reflex you can perform while still asleep. A mission needs your thinking brain switched on — and once it is on, sinking back under is much harder.

It survives every trick

Force-quit the app, restart the phone, flip it to silent, turn the volume down. The alarm re-arms and keeps going until the mission is actually done.

It gets you physically upright

Photo and barcode missions are anchored to a spot in another room, so the only way to stop the noise is to stand up and walk there. You cannot pre-stage it from bed.

It fixes the night, not just the morning

A sleep forecast before bed, bedtime reminders and wake-up streaks — because the 6 AM problem usually starts at 1 AM.

↑