What Time Should I Go to Bed? Using 90-Minute Sleep Cycles
Waking mid-cycle is why you can feel groggy after "enough" sleep. Time your bedtime to the end of a cycle instead.
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Ever slept a full eight hours and still woken up feeling like a truck hit you? The reason is often *when* in your sleep the alarm went off — not how long you slept. This is where sleep cycles come in.
Your sleep is not one flat block. It runs in cycles of about 90 minutes, and within each cycle you travel from light sleep, down into deep sleep, and up into dreaming (REM) sleep — a bit like a wave dipping down and rising back up. Wake up at the top of a wave (the end of a cycle, in light sleep) and you feel refreshed. Get yanked out from the bottom (deep sleep, mid-cycle) and you feel groggy and heavy for ages — even after a "full" night. It is the same reason being woken from your deepest sleep at 3 a.m. feels awful.
Most adults get 4–6 of these cycles a night, and around 5 cycles (about 7.5 hours) suits many people well. The trick is to aim your wake-up time for the *end* of a cycle. Add roughly 15 minutes for the time it takes to actually fall asleep, and you can simply count backwards from your alarm.
How to use it (with a worked example)
Set your wake-up time in the calculator above and it counts back in 90-minute cycles — plus 15 minutes to drop off — to suggest bedtimes that land you at the end of a cycle.
Here is how the maths works if you want to wake at 7:00 AM. Five cycles is 7.5 hours; add 15 minutes to fall asleep and you get 7 hours 45 minutes, so you would head to bed around 11:15 PM. Prefer more sleep? Six cycles is 9 hours; plus 15 minutes means going to bed around 9:45 PM. Both bedtimes line you up to wake naturally at the top of a cycle, rather than being dragged out of deep sleep.
One honest caveat: cycles *average* 90 minutes but really range from about 80 to 110 minutes, and they change through the night. So treat these times as helpful targets, not a stopwatch. If you wake 15 minutes off, that is completely normal.
Bedtime timing is only half the story
Cycle timing is a nice bonus, but do not let it distract you from the two things that matter more: getting enough total sleep and keeping a consistent schedule. A perfectly-timed 4.5 hours will still leave you exhausted, because it is simply not enough sleep.
So use the cycle trick on top of the basics, not instead of them: aim for your age-based sleep need, go to bed and wake up at roughly the same time every day (weekends included), and protect the wind-down hour before bed with dim lights and no doom-scrolling.
Frequently asked questions
What time should I go to bed to wake up at 6am?
Counting back in 90-minute cycles plus about 15 minutes to fall asleep: roughly 8:45 PM (6 cycles / 9 hrs), 10:15 PM (5 cycles / 7.5 hrs) or 11:45 PM (4 cycles / 6 hrs). Five cycles suits most adults. Use the calculator above for any wake time.
How long is one sleep cycle?
About 90 minutes on average, though it ranges from roughly 80 to 110 minutes and gets longer as the night goes on. Within each cycle you move from light sleep, into deep sleep, and up into REM (dreaming) sleep.
Is it better to wake at the end of a sleep cycle?
Yes — waking at the end of a cycle, when you are in lighter sleep, tends to feel much less groggy than being pulled out of deep sleep mid-cycle. That is why a "full" eight hours can still leave you feeling terrible if the alarm caught you at the wrong moment.
Why do I feel worse after 8 hours than after 6 sometimes?
Because the alarm caught you mid-cycle, deep in sleep, on the 8-hour night, but at the end of a cycle on the 6-hour night. Total hours matter, but being woken from deep sleep makes you groggy regardless of how long you slept. Aiming your wake time at the end of a cycle helps avoid it.
Related guides
Formulas are verified against official or authoritative sources and reflect rules known as of 9 July 2026. Universities can revise conversion rules — always confirm with your examination cell for official submissions.