Sleep physiology
Rest, energy, mood and metabolism
Why sleep matters
Key takeaway for patientsSleep is part of how the body regulates energy, appetite and mood. Persistent poor sleep is a health matter, and it can be assessed.
Sleep is an active state in which the body and brain do work they cannot do while awake. Hormones are released on a nightly schedule, tissue is repaired, the immune system is tuned, and the brain consolidates memory and clears metabolic waste.
Sleep is timed by two processes: a pressure to sleep that builds through the day, and the circadian clock, which is set mainly by light. When sleep is short or broken, the effects reach appetite, blood sugar, mood and concentration within days.
Key functions of sleep
- Hormonal regulation
- Growth hormone rises and cortisol falls in the first half of the night.
- Metabolic regulation
- Sleep restores insulin sensitivity and the balance of the appetite hormones leptin and ghrelin.
- Brain maintenance
- Memory is consolidated and waste is cleared.
- Mood and emotional regulation
- REM sleep helps to process emotional experience.
Factors that disturb sleep
Sleep is easily disturbed, from outside and from within:
- Irregular hours and light at night
- Shift the circadian clock.
- Caffeine, alcohol and late meals
- Delay sleep or fragment it.
- Stress and low mood
- Keep the brain alert when it should wind down.
- Sleep disorders and medical conditions
- Sleep apnoea, pain, thyroid disease and the menopause among them.
Clinical management and physician guidance
Poor sleep is assessed for its pattern and for the conditions behind it.
- Sleep assessment
- Sleep history and diary, screening for sleep apnoea, and blood tests where indicated.
- Physician-guided care
- Sleep habits and the care of any underlying condition, with referral for a sleep study where needed.
- Monitoring and follow-up
- Symptoms and markers are reviewed over time, and the plan is adjusted to them.
Research focus in sleep and health
| Core focus area | Biological target | Clinical relevance |
|---|---|---|
| Circadian timing | The suprachiasmatic nucleus | When sleep occurs |
| Metabolic effects | Insulin sensitivity after sleep loss | Blood sugar and weight |
| Appetite | Leptin and ghrelin | Hunger after short sleep |
| Brain maintenance | Slow-wave sleep | Memory and the clearance of waste |
Frequently asked questions
- How much sleep do adults need?
- Most guidance recommends at least seven hours a night for adults, though needs vary from person to person.
- Why am I hungrier after a bad night?
- Short sleep shifts the appetite hormones toward hunger.
- When should I speak to a physician?
- If poor sleep lasts more than a few weeks, or comes with loud snoring, pauses in breathing or sleepiness during the day.
Academic references and literature
- Spiegel K, Leproult R, Van Cauter E (1999). Impact of sleep debt on metabolic and endocrine function. Lancet, 354(9188), 1435-9. PMID 10543671.
- Watson NF et al. (2015). Recommended Amount of Sleep for a Healthy Adult: A Joint Consensus Statement of the American Academy of Sleep Medicine and Sleep Research Society. Sleep, 38(6), 843-4. PMID 26039963.
- Borbély AA et al. (2016). The two-process model of sleep regulation: a reappraisal. J Sleep Res, 25(2), 131-43. PMID 26762182.