Inside one night of sleep

A typical adult cycles through four stages of sleep roughly every 90 minutes, completing 4–6 cycles per night. The composition shifts: deep slow-wave sleep dominates early in the night, REM dominates toward morning. Each stage has distinct biological functions — cutting any of them short has consequences.

The four stages of sleep

N1 · LIGHT Falling asleep ~5% of total

The brief transition from waking to sleeping. Heart rate slows; muscles relax; you may experience hypnic jerks. Easily woken.

N2 · LIGHT Stable sleep ~45% of total

The largest portion of sleep. Body temperature drops, heart rate slows further. Sleep spindles and K-complexes consolidate memory and protect from waking.

N3 · DEEP Slow-wave (deep) sleep ~25% of total · early night

The most physically restorative stage. Growth hormone is released, immune memory is forged, and the glymphatic system clears β-amyloid and other metabolic waste from the brain.

REM · DREAM Rapid Eye Movement ~20–25% · later night

Brain activity rivals waking; eyes dart under closed lids; major muscles are paralysed (atonia). The substrate for vivid dreaming, emotional regulation, and creative problem-solving.

What chronic short sleep does Sleeping <6 hours regularly is independently associated with increased risk of cardiovascular disease, hypertension, type 2 diabetes, obesity, depression, anxiety, and reduced immune function. Cognitive performance after several nights of 5-hour sleep is comparable to mild alcohol intoxication — and the people most affected are usually the least aware of it.

How much you need

Nine evidence-based practices

Signs you may have a sleep disorder

  1. Loud habitual snoring with daytime sleepinessPossible obstructive sleep apnoea (OSA). An untreated independent risk factor for hypertension, atrial fibrillation, stroke, and accidents. Get a sleep study.
  2. Witnessed pauses in breathing during sleepStop. Get evaluated. CPAP and other treatments dramatically improve quality of life and reduce CV risk.
  3. Restless legs sensations preventing sleep onsetRestless legs syndrome — often treatable. Check iron (ferritin) levels; avoid caffeine and alcohol in the evening.
  4. Acting out dreams (kicking, shouting, falling out of bed)REM sleep behaviour disorder — speak with a clinician; can be an early marker of neurodegenerative disease.
  5. Chronic insomnia (≥3 nights/week for ≥3 months)The first-line treatment is Cognitive Behavioural Therapy for Insomnia (CBT-I), not sleeping pills. Effective and lasting.
  6. Excessive daytime sleepiness despite adequate sleep timeCould be narcolepsy, OSA, depression, or other causes. Worth investigating.

The seven-day sleep reset

If your sleep is poor, try this for one week before considering anything else:

  1. Fix the wake-up timeSame time every day, including weekends. The wake-up time anchors your circadian rhythm more reliably than the bedtime.
  2. Get bright morning light10–30 min of outdoor light within an hour of waking — even on cloudy days. The strongest signal to your internal clock.
  3. Cut caffeine after noonCaffeine has a 5–6 h half-life. A 3 p.m. coffee is still 25% in your system at midnight.
  4. Stop alcohol within 3 hours of bedAlcohol sedates but suppresses REM and fragments the second half of the night.
  5. Dim lights and screens 90 min before bedSwitch to warm, low light. Avoid distressing news and emotionally charged conversations.
  6. Cool the bedroom (~18°C / 65°F)Body temperature drops at sleep onset; a cool room facilitates that drop.
  7. If awake more than 20 min — leave the bedDo something calm in dim light until sleepy. Reserves the bed for sleep.
Track your sleep The Team21 Daily Tracker includes a "7–9 hours of sleep" habit. Across a week or month you will see whether sleep is consistently meeting your target.
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