"Why am I always sleepy in spring?" "Why is summer-night sleep so shallow?" These have specific physiological answers. Human sleep follows the four seasons, and each demands different responses.
Spring — daylight surge and "spring drowsiness"
Daylight changes at Korean latitude (~37°N):
| Month | Daylight | Change vs March |
|---|---|---|
| March | ~12 hours | — |
| April | ~13 hours | +1 hour |
| May | ~14 hours | +2 hours |
| June | ~14.5 hours | +2.5 hours |
2.5-hour increase in 3 months — a very fast change. The SCN (master clock) struggles to adapt to the new light pattern; melatonin timing wobbles, producing "spring drowsiness" — daytime sleepiness, delayed sleep onset, early-morning wake-ups.
Spring strategy:
- Deliberately increase morning light exposure (10+ minutes)
- Stricter evening light blocking (switch to warm indoor lighting)
- Sleep 30 min – 1 hour more for the first 1–2 weeks of spring
- Stricter caffeine cutoff (by 1 PM)
- 5-min post-lunch walk to manage afternoon sleepiness
Summer — heat and reduced deep sleep
Summer is the hardest sleep season. Reasons:
- When night temperatures stay above 25°C, core body temperature can't drop → less deep sleep
- Humidity above 70% prevents sweat evaporation → broken thermoregulation
- Longest daylight keeps it bright into the evening → delayed sleep onset
- Monsoon pressure changes cause headaches and sleep disruption for some
Korean summer average temperatures rose 1.5°C from the 1990s to 2010s — sleep problems are worsening over time.
Summer strategy:
- Bedroom under 26°C (run AC 1–2 hours before bed)
- Dehumidify (target under 60%)
- Breathable bedding (linen, Tencel, natural fibers)
- Cool shower (1 hour before bed, drops body temp fast)
- Reduce evening light (only 1 hour bright after sunset)
Autumn — the golden season for sleep
Autumn aligns most with the human circadian rhythm:
- Daylight stable around 12 hours — most consistent SCN signal
- Night temperatures 18–22°C — natural bedroom cooling
- Optimal humidity (40–60%)
- Abundant food — diet variety means nutritional balance
Korean statistics show autumn (Sep–Nov) average sleep is ~20 minutes longer than other seasons, with the highest deep-sleep ratio.
Autumn strategy:
- Use this period to recover sleep debt
- Prepare a light therapy lamp for winter (Oct–Nov)
- Increase exercise (coolest, safest season)
Winter — short daylight and SAD
Winter daylight ~9.5 hours (December). Short, weak sun weakens the SCN signal:
- Hard to wake early (melatonin lingers later)
- Strong afternoon sleepiness
- Sleep onset is faster but quality unchanged
- Some (~5–10%) develop SAD (Seasonal Affective Disorder) — depression + hypersomnia + simple-carb cravings
SAD is less common in Korea than in the US or Northern Europe but does exist, more common in higher-latitude regions like Gangwon and northern Gyeongbuk.
Winter strategy:
- Light therapy lamp: 10,000 lux for 30 morning minutes — most effective single tool for SAD prevention and circadian stability
- Morning outdoor activity: cloudy days still 1,000+ lux
- Vitamin D supplementation: 80%+ of Koreans are vitamin D deficient in winter — 1,000–2,000 IU daily
- Maintain exercise: indoor exercise at minimum
- Heating temperature management: bedroom 18–20°C (too warm reduces deep sleep)
- Suspected SAD: see a psychiatrist. Light therapy + low-dose antidepressants effective
Monsoon — Korea-specific challenge
Late June – early July monsoon is a special Korean sleep situation. 80%+ humidity + pressure changes + typhoons + darkness = every sleep-degradation factor at once.
Monsoon strategy:
- Aggressive dehumidifier use (don't skimp on electricity)
- Mold prevention — regular mattress and pillow ventilation
- Substitute light source for low natural sunlight
- Rain sounds work as white noise — actually helpful
Season-transition adaptation (especially spring↔summer, autumn↔winter)
The hardest periods are April (spring → early summer) and October (autumn → early winter). Daylight changes fastest and temperature swings are large.
Principles:
- Adapt gradually over 2 weeks — avoid sudden schedule changes
- Wake-time consistency during transitions (strongest circadian signal)
- Sleep 30 minutes more in the first week if possible
- Stricter caffeine during transitions
Track your seasonal patterns
Mapping your sleep over a full year gives the most precise response.
- Monthly average sleep duration (tracker data)
- Monthly sleep onset time
- Monthly nighttime awakening frequency
- Monthly daytime sleepiness frequency
After a year, knowing your hardest and easiest months lets you prepare next year.
Conclusion — flow with nature, not against it
Seasonal sleep changes are natural human adaptation. Don't fight them — flow with them. Sleep a little more in spring, stay cooler in summer, recover deeply in autumn, deliberately seek light in winter. The same person becomes a different person each season — that's your body's wisdom.
