Sleep: what the stages are for, and what goes wrong

The advice is always about hours. Get seven to nine. The number is not wrong,
but it hides the thing that actually matters, which is that sleep is not one
state you are either in or out of. It is a cycle through several distinct
states, each doing different work, and most of what goes wrong is a failure to
complete that cycle rather than a shortage of time spent in bed.

What the night is made of

Your brain moves repeatedly between two broad kinds of sleep, in cycles of
roughly ninety minutes.

Non-REM sleep has progressively deeper stages. The deepest is where the
body does most of its physical repair, and where growth hormone is released.
It is also the hardest to be woken from — someone roused out of deep sleep is
groggy and disoriented for several minutes.

REM sleep is where most vivid dreaming happens. The brain is close to as
active as when awake, while the body is temporarily paralysed. It appears to
matter for memory consolidation and emotional processing.

The proportions shift across the night. Deep sleep is concentrated early; REM
periods get longer towards morning. That has a practical consequence: cutting
your night short at the end does not remove a slice of average sleep — it
removes most of your REM.

Why quality is not the same as quantity

If something interrupts you repeatedly, you can spend eight hours in bed and
still not complete enough cycles. Each awakening, even one too brief to
remember, can drop you back to a lighter stage and restart the climb.

This is why people with sleep apnoea often insist they slept all night and
still cannot stay awake the next day. They did sleep all night. They did not
get through the stages.

The main disorders

• Insomnia — difficulty falling or staying asleep. The most common, and
  the one most likely to be self-treated with alcohol, which suppresses REM
  and makes it worse.
• Sleep apnoea — breathing repeatedly stops, and each episode wakes you
  briefly. Frequently undiagnosed for years, because the sufferer has no memory
  of the awakenings.
• Restless legs syndrome — an urge to move the legs that gets worse at
  rest, delaying sleep onset.
• Narcolepsy — a disorder of the boundary between wake and REM, producing
  sudden daytime sleep and sometimes muscle collapse triggered by emotion.
• Circadian rhythm disorders — the internal clock is out of step with the
  schedule required, as in shift work or jet lag.

What the evidence supports

For chronic insomnia, cognitive behavioural therapy for insomnia (CBT-I)
outperforms sleeping pills in the long run, and the guidelines now say so.
The usual first-line advice still holds: a consistent schedule including
weekends, a dark and cool room, no caffeine late, and getting out of bed if
sleep does not come rather than lying there associating the bed with being
awake.

For apnoea the treatment is mechanical rather than behavioural, which is why
diagnosis matters: no amount of sleep hygiene fixes an airway that closes.

When to see someone

If poor sleep has lasted more than a month, if you are sleepy enough during the
day that it affects driving or work, or if anyone has told you that you stop
breathing at night. The last one in particular is worth acting on quickly.