The Basics: How Sleep Works
Sleep is not a passive, uniform state — it's a highly structured biological process cycling through stages every approximately 90 minutes. Each full cycle includes light sleep (stages 1–2), deep slow-wave sleep (stage 3) and REM (Rapid Eye Movement) sleep.
- Deep sleep (stage 3): Physical restoration, growth hormone secretion, immune system strengthening. This stage predominates in the first half of the night.
- REM sleep: Memory consolidation, emotional processing, creativity. REM periods lengthen in the second half of the night — cutting sleep short disproportionately reduces REM.
Waking during deep sleep causes sleep inertia — that groggy, disoriented feeling lasting 20–30 minutes. Waking at the end of a complete cycle feels dramatically more refreshed.
How Many Hours Do You Need?
• Teenagers (14–17): 8–10 hours
• Young adults (18–25): 7–9 hours
• Adults (26–64): 7–9 hours
• Older adults (65+): 7–8 hours
Individual variation exists — a small percentage of people genuinely function well on 6 hours ("short sleepers"), but most people who believe they've adapted to sleep deprivation are simply experiencing chronic impairment they've normalised.
Sleep and Weight: The Underappreciated Connection
The relationship between sleep and weight management is bidirectional and powerful:
- Ghrelin and leptin: Just one night of poor sleep increases ghrelin (the hunger hormone) by up to 24% and decreases leptin (the satiety hormone) by 18%. This translates directly to increased appetite — especially for high-calorie, high-carbohydrate foods.
- Insulin sensitivity: Sleep deprivation reduces insulin sensitivity by up to 25%, increasing fat storage and risk of type 2 diabetes.
- Cortisol: Poor sleep elevates cortisol, promoting visceral fat accumulation.
- Metabolism: Chronic sleep restriction (6 hours/night) reduces resting metabolic rate.
Studies show that people sleeping less than 6 hours per night have a 55% higher risk of obesity compared to those sleeping 7–9 hours.
Practical Tips for Better Sleep
- Consistent schedule: Going to bed and waking at the same time every day (including weekends) is the single most impactful sleep habit.
- Cool bedroom: Core body temperature drops during sleep — a room temperature of 16–19°C (60–67°F) is optimal for most people.
- Light exposure: Get bright light in the morning to anchor your circadian rhythm. Avoid blue light (screens) for 1–2 hours before bed.
- Caffeine cutoff: Caffeine has a half-life of 5–6 hours — a 3pm coffee still has 50% of its stimulant effect at 8pm. Move your last caffeine to before noon if sleep is a problem.