Enter the time you have to be up. These are the bedtimes that give you whole sleep cycles rather than a broken last one.
Go to bed at
five full cycles before a 6:30 am alarm — 7 h 30 m of sleep
4 cycles · 6 h 00 m12:16 am
5 cycles · 7 h 30 m10:46 pm
6 cycles · 9 h 00 m9:16 pm
Alarm
6:30 am
Asleep by
11:00 pm
Cycle length
90 minutes
This calculator needs JavaScript to recalculate. The answer shown above was worked out when the page was built and is correct for the date in the address bar.
Working backwards, and allowing 14 minutes to fall asleep:
Cycles
Time asleep
Get into bed at
6
9 h 00 m
9:16 pm
5
7 h 30 m
10:46 pm
4
6 h 00 m
12:16 am
Note that these are times to be in bed with the light off, not times to start getting ready. If your evening routine takes half an hour, the 10:46 pm row means starting at about quarter past ten.
Why work backwards
Bedtime is the variable you actually control. The alarm is fixed by work, school or a train, and no amount of planning moves it. So the useful question is almost never “when will I wake up” — it is “how late can I stay up and still get five clean cycles”. That is what this page answers.
The other reason to work backwards: a fixed wake time is the single most effective thing most people can do for their sleep, far more so than hitting a cycle boundary. Keeping the alarm constant and letting bedtime move is the version of this that survives contact with real life.
Methodology
Bedtime = wake time − (90 minutes × number of cycles) − 14 minutes for sleep onset. Cycles are modelled at the conventional 90-minute average.
Limitations
Real cycles run roughly 70 to 120 minutes and are not the same length all night, so a bedtime six cycles out is a much rougher target than one four cycles out. The 14-minute onset allowance is an average and will be wrong on any night you are wired, ill or have had a late coffee. Total sleep matters more than boundary timing.
This page does arithmetic on sleep cycles. It is not medical advice and nothing here is a claim about health.
Common questions
What time should I go to bed to wake up at 6 am?
For five cycles, 10:16 pm; for six, 8:46 pm; for four, 11:46 pm. Those already include 14 minutes to fall asleep, so they are times to have the light off, not times to start brushing your teeth.
Should I aim for the same bedtime every night?
A consistent wake time does more than a consistent bedtime, and it is the one most people can actually hold. If you are going to fix one of the two, fix the alarm and let bedtime move within an hour or so. This calculator is built around that assumption, which is why it works backwards.
It is already past the bedtime you suggested. What now?
Take the next row down. Four cycles — six hours — still ends on a boundary. If you are past that too, go to bed anyway; the amount of sleep matters more than where the alarm lands, and staying up an extra 40 minutes to hit a cycle boundary is a bad trade.
Does the 14 minutes include reading in bed?
No. It is the time between deciding to sleep and actually being asleep. If you read for 25 minutes first, add that to your bedtime — or set the sleep-onset field to 39 minutes, which produces the same answer.
Why does going to bed earlier sometimes feel worse?
Usually because you woke mid-cycle rather than at the end of one, which the six-cycle row is more prone to — cycles lengthen through the night, so the further out the boundary, the wider the error on a 90-minute average. It is also entirely possible to have simply gone to bed before you were tired and spent the extra time awake. Neither is something this calculator can detect.