What Time Should I Go to Bed? Sleep Cycles, Explained
The quick version: decide what time you have to get up, count backwards in blocks of 90 minutes, then add about fifteen minutes for falling asleep. That is your bedtime. Aim for five or six blocks.
The rest of this article is why that works, and — more usefully — where it stops being true. The rule is a good average dressed up by most websites as a law, and knowing the difference will save you from optimising the wrong thing.
The short answer, in a table
If you need to be awake at 7:00 am:
| Cycles | Sleep | Asleep by | In bed at |
|---|---|---|---|
| 6 cycles | 9 hours | 10:00 pm | 9:45 pm |
| 5 cycles | 7 h 30 | 11:30 pm | 11:15 pm |
| 4 cycles | 6 hours | 1:00 am | 12:45 am |
| 3 cycles | 4 h 30 | 2:30 am | 2:15 am |
For any other wake time, the arithmetic is the same: subtract 90 minutes per cycle, then subtract fifteen more. Five cycles is the realistic weeknight target for most adults. Six is the one you should aim for when you have been short for several nights running.
Where the 90 minutes comes from
Sleep is not a single flat state you sink into and climb out of eight hours later. It is a repeating loop. Across a normal night you go round that loop four to six times, and each lap takes roughly an hour and a half.
Each lap moves through four stages:
- N1 — the doorway. A few minutes of drifting, easily interrupted, often not remembered as sleep at all.
- N2 — light sleep, and the largest share of the night. Body temperature drops, heart rate slows.
- N3 — deep or slow-wave sleep. The hardest stage to be woken from, and the one associated with physical recovery.
- REM — rapid eye movement. Brain activity close to waking levels, vivid dreams, the body temporarily paralysed.
The important detail is that the laps are not identical. Deep sleep is front-loaded and REM is back-loaded. Your first two cycles carry most of the night's N3; by the small hours, N3 has nearly disappeared and REM periods have stretched out. This is why the last two hours of a night feel dream-heavy, and why cutting a night short does not simply remove a proportional slice of everything — it removes almost all of your REM and very little of your deep sleep.
Why the end of a cycle is the easy place to wake up
At the end of each lap, sleep briefly lightens — often to the point of a short, unremembered awakening. Waking there is easy because you were nearly awake anyway.
Waking in the middle of N3 is the opposite experience. Your brain has to climb out of its slowest, least responsive state and does not manage it instantly. The result is sleep inertia: the heavy, stupid, mildly hostile feeling most people know as waking up on the wrong side of the bed. It typically clears in fifteen to sixty minutes, and during that window your reaction time and judgement are measurably worse than they will be an hour later.
That is the entire mechanism behind every sleep calculator, including this one. It is not trying to give you more sleep. It is trying to land your alarm in a shallow stretch instead of a deep one.
The honest limits of the rule
Now the part most articles on this subject leave out.
Ninety minutes is an average, not your number
Measured cycle lengths run from about 70 to 120 minutes depending on the person. The first cycle of the night tends to be the shortest, often 70 to 100 minutes, and later cycles run longer. So a 90-minute model that is accurate at cycle one may be forty minutes out of phase by cycle five.
The practical consequence: treat the calculated time as the centre of a window, not a target to hit to the minute. Being ten minutes either side of it changes nothing. Setting an alarm for 6:47 am rather than 6:45 am is superstition.
Total sleep beats alignment
If the choice is between 7 h 30 that lands neatly on a boundary and 8 h 15 that does not, take the sleep. Cycle alignment shaves some grogginess off the first hour of your morning. Chronic short sleep costs you considerably more than that, all day, every day. Never cut an hour of sleep to hit a cycle boundary. The rule is for choosing between bedtimes you were already considering.
The fifteen minutes is a guess about you
Every calculator assumes a fixed time to fall asleep. Fifteen minutes is the usual figure and a reasonable default, but yours may be five or forty, and it changes with stress, caffeine, exercise and how short you were the night before. If you consistently lie awake for half an hour, put half an hour in — the calculator is only as good as that one assumption.
How long should falling asleep take?
Ten to twenty minutes is typical and healthy. Two useful signals sit either side of it.
Under five minutes, every night, is not a sign of being a great sleeper. It usually means significant sleep debt — you are falling asleep that fast because you are that far behind.
Over thirty minutes, most nights, for more than a few weeks, is worth taking seriously rather than solving with a calculator. Persistent difficulty falling or staying asleep is the definition of insomnia, and it responds well to treatment — but not to arithmetic about cycles.
How many cycles do you actually need?
Adults generally need seven to nine hours, which is five to six cycles. Teenagers need more, typically eight to ten. The "I only need five hours" claim is almost always someone who has adapted to feeling worse and forgotten the comparison.
Four cycles — six hours — is a survivable night, not a plan. Three is triage. If you find yourself building weeks out of four-cycle nights and compensating at the weekend, the problem is the schedule, and no amount of clever alarm timing will fix it.
The other direction: from bedtime to wake time
Sometimes the fixed point is the other end. You are going to bed now, and the question is when to set the alarm. Same arithmetic, forwards: add fifteen minutes, then add cycles.
Going to bed at 11:00 pm gives you 6:15 am for five cycles, or 7:45 am for six. This is the more useful mode on a night that has already got away from you, and the calculator works both ways for exactly this reason.
If you only have five hours
Five hours is not a whole number of cycles, so you have a choice: 4 h 30 (three cycles) or 6 h (four). If your alarm is fixed and it is nearly midnight, the honest advice is to go to bed now and take whatever you get — the fifteen minutes you would spend calculating is worth more asleep.
Where the choice genuinely exists, three clean cycles usually beat five ragged hours. But this is a one-off tactic for a bad night, not a system. The research on short sleep is unambiguous and it does not care how elegantly your alarm was timed.
Naps: twenty minutes or ninety, nothing in between
The same architecture explains why naps are so unpredictable.
Twenty minutes keeps you in N1 and N2. You wake easily and get a genuine alertness boost, best taken in the early afternoon.
Ninety minutes takes you through a full cycle and lets you surface at the shallow end. Restorative, but it will push your bedtime back.
Forty-five minutes is the trap. It is long enough to drop you into deep sleep and short enough to yank you out of it, which is why a "quick" nap sometimes leaves you worse than before. If you cannot commit to a full cycle, keep it short.
Four things that matter more than cycle timing
- A consistent wake time. Your body clock is anchored by when you get up and see light, far more than by when you go to bed. Fixing the wake time and letting bedtime drift works better than the reverse.
- Morning daylight. Ten minutes outside shortly after waking does more for tonight's sleep than anything you can do at 11 pm.
- Caffeine's half-life. Roughly five to six hours, which means a 4 pm coffee is still half-present at 10 pm. If you cannot fall asleep, this is the first thing to test.
- Alcohol. It shortens the time to fall asleep and wrecks the second half of the night, suppressing REM and fragmenting sleep. It is the most common reason for waking at 4 am feeling unrested after eight hours in bed.
Common questions
Is 7 hours 30 really better than 8 hours?
Sometimes, if the eight-hour version drops your alarm into deep sleep. But the difference is one groggy morning half-hour against thirty extra minutes of sleep, and it depends on a cycle length that is only estimated. When the two are close, take the sleep.
Why do I wake up naturally just before my alarm?
Because your body anticipates a consistent wake time and starts lightening sleep in advance. It is a good sign, and the strongest argument for keeping the same wake time at weekends.
Do sleep-tracking apps measure my cycles properly?
They estimate from movement and heart rate, which correlates with sleep depth but does not measure it. Useful for spotting trends over weeks; not accurate enough to justify moving your alarm by ten minutes.
Does the 90-minute rule work for children?
Not as written. Young children have shorter cycles — around 50 to 60 minutes in infancy — lengthening towards the adult figure through childhood. The counting-backwards method still works; the block size does not.
The short version
Pick your wake time. Subtract fifteen minutes for falling asleep, then subtract 90 minutes per cycle, aiming for five or six. Treat the result as the middle of a twenty-minute window rather than an exact instant. Keep the wake time steady, get light in the morning, and never trade an hour of sleep for a tidier number.
The sleep cycle calculator does the arithmetic in both directions — from a wake time or from a bedtime — and turns any of the results into an alarm with one tap, so you do not have to do subtraction at midnight.