What Time Should I Go to Bed? Sleep Cycles Explained

Why Eight Hours Can Feel Worse Than Seven and a Half

Most people set a bedtime by counting hours backwards from the alarm. That ignores the thing that actually decides how you feel at 7am: which stage of sleep your alarm interrupts.

Sleep is not a flat state you sink into and climb out of. It moves in repeating cycles, and being pulled out of the deepest part of one produces sleep inertia — the heavy, disoriented fog that can last anywhere from fifteen minutes to well over an hour. Wake at the end of a cycle instead, when sleep is at its lightest, and the same alarm feels far kinder.

What a Sleep Cycle Contains

A full cycle runs through several stages:

N1 — the brief drift from wakefulness into sleep, only a few minutes long.
N2 — light sleep, where you spend roughly half the night.
N3 — deep or slow-wave sleep. This is the physically restorative stage, and the one that produces the worst grogginess if you are woken from it.
REM — rapid eye movement sleep, where most vivid dreaming happens and where memory consolidation is thought to occur.

The mix shifts as the night goes on. Early cycles are heavy on deep sleep; later cycles are heavy on REM. That is why cutting your night short costs you disproportionately more REM, and why a late night followed by an early alarm leaves you feeling mentally rather than just physically flat.

Working Backwards From Your Alarm

The calculation uses two numbers: a cycle of about 90 minutes, and roughly 14 minutes to actually fall asleep once the light is off.

Bedtime = Wake Time − (Cycles × 90 minutes) − 14 minutes

If your alarm is set for 7:00am:

6 cycles (9 hours of sleep) — go to bed at 9:46pm
5 cycles (7.5 hours) — go to bed at 11:16pm
4 cycles (6 hours) — go to bed at 12:46am
3 cycles (4.5 hours) — go to bed at 2:16am

It works in the other direction too. Turning in at 11:00pm puts a natural waking point at 6:44am after five cycles, or 8:14am after six.

How Many Cycles Should You Aim For?

Adults generally need seven to nine hours, which lands neatly on five or six complete cycles. Five cycles gives 7.5 hours and six gives 9.

The practical consequence is that when you are short on time, finishing a cycle beats squeezing in extra minutes. Six hours ending cleanly between cycles will usually leave you sharper than six hours and forty minutes that cuts through deep sleep.

Where This Stops Being Science

Sleep calculators rarely admit this, so here it is plainly: 90 minutes is an average, not a rule.

Real cycles run roughly 70 to 120 minutes, and vary between people.
They also vary within a single night for the same person, tending to lengthen towards morning.
The 14-minute allowance is a typical healthy sleep latency. If you routinely take 45 minutes to drop off, your bedtime needs to move earlier by the difference.

So treat the output as a sensible target rather than a precise instruction. If you consistently wake groggy at a suggested time, shift your bedtime by fifteen or twenty minutes and try again for a week — you are calibrating an average to your own cycle length.

Falling Asleep Too Fast Is Not a Good Sign

A healthy sleep latency is somewhere around 10 to 20 minutes. Regularly losing consciousness within five minutes of your head touching the pillow is usually a marker of sleep deprivation rather than good sleep hygiene, and the fix is more sleep rather than a cleverer bedtime.

The Caffeine Arithmetic

Caffeine has a half-life of roughly five hours in most adults, meaning half of it is still circulating five hours after your last cup. Working it forward from a single 95 mg coffee at 3pm:

6pm: around 63 mg remaining
8pm: around 48 mg remaining
11pm: around 31 mg remaining

A third of an afternoon coffee is still active at bedtime, which is enough to lengthen how long you take to fall asleep and to reduce deep sleep even when you do drop off. If your bedtime maths keeps failing, the cause is often in the afternoon rather than the evening.

What Matters More Than Cycle Timing

Cycle arithmetic is a useful nudge, but three habits outrank it:

Consistency. The same bedtime and wake time every day, weekends included, does more for sleep quality than any single well-timed night.
Morning light. Daylight soon after waking anchors your body clock and makes the following night easier.
Alcohol. It shortens the time to fall asleep and then fragments the second half of the night, suppressing REM. A nightcap trades sleep quality for sleep speed.

When to Speak to Someone

No bedtime calculation fixes a sleep disorder. If you regularly struggle to fall or stay asleep for a month or more, wake unrefreshed despite adequate hours, snore loudly, or have been told you stop breathing in your sleep, that is worth raising with a GP. Insomnia and sleep apnoea are common and treatable, and neither responds to better arithmetic.

Conclusion

Count cycles rather than hours, add roughly a quarter of an hour to fall asleep, and aim for five or six complete cycles. Then adjust from what you actually feel, because 90 minutes is your starting estimate rather than your personal figure. Put your wake time into a Sleep Cycle Calculator to get the bedtimes worked out for you — and if one of them keeps leaving you groggy, move it by fifteen minutes and try the next.

Try our Sleep Cycle Calculator