Sleep cycles: how long is a sleep cycle?
Last reviewed: 14 September 2026
A sleep cycle averages about 90 minutes, but the interesting part is not the length — it is how completely the makeup of each cycle changes between the start of the night and the end.
About 90 minutes — but it varies
A sleep cycle lasts roughly 90 minutes, and you go through four to six of them in a normal night. The 90-minute figure is an average: the first cycle is usually the shortest at around 70 to 100 minutes, and later cycles run to 120.
What changes across the night matters more than the length. Deep sleep is packed into the first two cycles; REM sleep is packed into the last two. That single fact explains most of what people find puzzling about sleep.
What a night actually looks like
This is a hypnogram — the chart sleep laboratories produce from an overnight recording. It shows a typical eight-hour night in a healthy adult.
The four stages of sleep
Sleep is divided into rapid eye movement (REM) sleep and three stages of non-REM sleep. A cycle runs N1 → N2 → N3 → back up to N2 → REM, then begins again.
| Stage | Typical length | Share of the night | What happens |
|---|---|---|---|
| N1 — drifting off | 1–7 minutes | About 5% | The handover from wake to sleep. Muscles relax, and you can be woken easily — people roused from N1 often deny they were asleep at all. |
| N2 — light sleep | 10–25 minutes, lengthening through the night | About 45% | The bulk of your night. Heart rate and body temperature drop. Bursts of brain activity called sleep spindles appear, and are strongly linked to consolidating what you learned during the day. |
| N3 — deep or slow-wave sleep | 20–40 minutes in early cycles, then shorter | About 25% | The hardest stage to be woken from, and the one that produces the physical restoration people mean by "deep sleep". Growth hormone is released, and the immune system does a lot of its work here. |
| REM — dreaming sleep | 10 minutes in the first cycle, up to an hour in the last | About 25% | Brain activity close to waking levels, with the body temporarily paralysed. Most vivid dreaming happens here, along with emotional processing and a large share of memory consolidation. |
Why waking between cycles feels better
Waking is not a switch. Being pulled out of N3 deep sleep produces sleep inertia — the heavy, disoriented grogginess that can take 20 to 30 minutes to clear, and considerably longer if the sleep was short. Waking from light N2 sleep, or from the end of a REM period, produces far less of it.
That is the whole logic behind timing an alarm to land near the end of a cycle rather than in the middle of one. It is a real effect, but it is worth being clear about its size: it changes how the first half-hour of your morning feels. It does not change how much sleep you got, and it is not a reason to cut an hour off the night to hit a neat multiple of 90 minutes. Total sleep beats cycle arithmetic every time.
Because real cycles range from about 70 to 120 minutes and vary from night to night, no calculator can tell you precisely where your cycle boundaries will fall. What a bedtime calculator gives you is a set of sensible target times that are more likely than not to land near a boundary — useful, but an average rather than a measurement.
How the cycles change as the night goes on
- Cycles 1 and 2 are deep-sleep heavy. If you sleep only four hours you get most of your N3 and almost none of your REM — which is why a short night tends to hit mood and memory harder than it hits physical energy.
- Cycles 3 and 4 are the transition. N3 shrinks, REM periods lengthen.
- Cycles 5 and 6 are almost entirely N2 and REM. Cutting an hour off the end of your night removes a disproportionate amount of REM sleep.
This is also why alcohol is so costly. It gets you to sleep faster and increases deep sleep early on, then suppresses REM and fragments the second half of the night — taking away precisely the part of the night that was going to be REM-rich.
Each stage has its own page, if you want the detail: deep sleep, REM sleep, and core sleep — the name Apple gives to N1 and N2.
What changes your cycle length
- Age. Children have longer cycles and far more deep sleep. From middle age onwards N3 declines markedly, which is a large part of why sleep feels lighter with age.
- Sleep debt. After deprivation the body prioritises N3 first, then REM, rebounding on each in turn over subsequent nights.
- Alcohol and cannabis both suppress REM; stopping either after regular use produces a REM rebound with unusually vivid dreams.
- Some antidepressants suppress REM sleep substantially.
- Temperature. A room that is too warm fragments deep sleep more than almost any other environmental factor.
Sources. Age-based recommendations are from the National Sleep Foundation’s sleep duration recommendations and the American Academy of Sleep Medicine’s consensus statements on sleep for healthy children and for adults. Sleep stage figures are from published polysomnography data. This page is general wellbeing information, not medical advice — speak to a GP or doctor about persistent sleep problems.
Common questions
How long is a sleep cycle?
About 90 minutes on average. In practice the first cycle of the night is usually the shortest, at around 70 to 100 minutes, and later cycles run between 90 and 120 minutes.
How many sleep cycles do you need a night?
Four to six is typical. Five cycles works out at roughly seven and a half hours, which sits nicely inside the 7 to 9 hours recommended for adults.
How long is a REM cycle?
REM is a stage within a cycle rather than a cycle of its own. The first REM period of the night lasts around ten minutes; they lengthen with each cycle, and the last can run close to an hour. REM makes up about a quarter of a night's sleep in total.
What are the 4 stages of sleep?
N1 (drifting off, about 5% of the night), N2 (light sleep, about 45%), N3 (deep or slow-wave sleep, about 25%) and REM (dreaming sleep, about 25%). A cycle runs N1 to N2 to N3, back up to N2, then REM.
Is it better to wake up at the end of a sleep cycle?
It generally feels better, because waking out of deep N3 sleep produces stronger sleep inertia — the grogginess that takes 20 to 30 minutes to clear. It is a real effect but a modest one, and it is not worth cutting your total sleep short to achieve.
Which cycle has the most deep sleep?
The first two. Deep sleep is heavily front-loaded — in a typical eight-hour night there is almost no N3 left after about five and a half hours, by which point the night has shifted almost entirely to N2 and REM.