Good if they are short and not too late. A long nap, or one after mid-afternoon, spends sleep pressure meant for the night. For someone currently dealing with insomnia, daytime sleep is usually dropped entirely during treatment.

Are the sleep stages on a smartwatch accurate?

Only as a rough guide. Consumer devices infer stages from heart rate and movement, and they are least accurate in exactly the people who have insomnia: lying still without sleeping is very easily read as sleeping.

I am a natural night owl. Can that be changed?

Chronotype is partly genetic, but it can be shifted with light at the right time and a gradual change of schedule. This is a circadian rhythm problem rather than insomnia, so the approach is different.

How sleep works

Name: How sleep works
Published: 2026-11-03

The two systems that drive sleep, the stages within a cycle, how much sleep is enough, and the things most often said wrongly about sleep.

Updated 2026-11-03

Key points

Sleep is driven by TWO systems: sleep pressure, which builds with hours awake, and the body clock, which sets when sleep comes easily.
Understanding these two systems is understanding why behavioural therapy works, because each technique targets one of them.
A cycle lasts around 90 minutes, but that is a POPULATION AVERAGE. In practice it ranges from 70 to 120 minutes and changes within a single night.
"You must get eight hours" is a population average, not a personal target. The right measure is whether you are alert during the day.

The two systems

Sleep pressure

The longer you stay awake, the more sleep pressure builds, and sleep discharges it. This explains why a long daytime nap makes the night harder: that nap spent pressure that was meant for the night. It also explains why spending longer in bed does not produce more sleep, because time lying awake builds nothing.

The body clock

Alongside sleep pressure, the body runs a roughly 24-hour rhythm that decides when in the day sleep comes easily and when you are at your most alert. The strongest signal anchoring that rhythm is LIGHT, and morning light above all. That is why a fixed WAKE time matters more than a fixed bedtime: the wake time decides when you get your light.

What a cycle contains

Stage	Share of the cycle	Role
N1, light sleep	The first few minutes	The transition between wake and sleep; easily roused
N2, stable light sleep	The largest share of sleep time	Associated with memory consolidation
N3, deep sleep	Most of it in the EARLY cycles of the night	Physical restoration; hardest to rouse from
REM sleep	Most of it in the LATE cycles, towards morning	Vivid dreams; associated with emotional processing

This uneven distribution is why waking 2 hours earlier than usual costs you mainly REM, while going to bed 2 hours later costs you mainly N3.

How much sleep is enough

The recommendation for adults is 7 hours or more a night on a regular basis. But that is a population figure, and individual need genuinely varies: some people need 6, some need 9. The practical measure is not the number on the clock but whether you are alert during the day, able to work, and not relying on caffeine to get through.

Four things often said wrongly

"You must get eight hours or you are in trouble." This belief by itself raises anxiety about sleep, and anxiety about sleep is one of the mechanisms that keeps insomnia going. It is on the list that cognitive therapy works on.
"Six hours of complete cycles beats seven hours of broken ones." There is no good evidence for this. Total duration is the stronger factor.
"Catching up at the weekend pays off the sleep debt." It pays part of it, but sleeping in at the weekend shifts the body clock and makes Sunday night harder.
"Alcohol helps you sleep." Alcohol shortens the time to fall asleep but fragments the second half of the night, reduces REM and causes awakenings. Sleep after drinking is less restorative.

Frequently asked questions

Are naps good?: Good if they are short and not too late. A long nap, or one after mid-afternoon, spends sleep pressure meant for the night. For someone currently dealing with insomnia, daytime sleep is usually dropped entirely during treatment.
Are the sleep stages on a smartwatch accurate?: Only as a rough guide. Consumer devices infer stages from heart rate and movement, and they are least accurate in exactly the people who have insomnia: lying still without sleeping is very easily read as sleeping.
I am a natural night owl. Can that be changed?: Chronotype is partly genetic, but it can be shifted with light at the right time and a gradual change of schedule. This is a circadian rhythm problem rather than insomnia, so the approach is different.

References

Recommended Amount of Sleep for a Healthy Adult: A Joint Consensus Statement - AASM & Sleep Research Society, 2015 · DOI 10.5664/jcsm.4758
How Sleep Works - Sleep Phases and Stages - National Heart, Lung, and Blood Institute (NIH), 2026
Behavioral and psychological treatments for chronic insomnia disorder in adults: an AASM clinical practice guideline - American Academy of Sleep Medicine, 2021 · DOI 10.5664/jcsm.8986