How do seasons affect your sleep?

Before electric light, humans slept differently in winter than summer. That biology hasn't changed — but our schedules pretend it doesn't exist.

The Biology of Seasonal Sleep

### Light as the Master Signal

Summer (long days): - Dawn at 5-6 AM → early cortisol rise → natural early waking - Melatonin onset delayed (bright evening light suppresses it) - Sleep pressure builds more slowly (more outdoor light = more alertness) - Natural sleep window narrows: 7-7.5 hours feels sufficient

Winter (short days): - Dawn at 7-8 AM → delayed cortisol → harder to wake - Melatonin onset earlier (dark evenings trigger it sooner) - Melatonin offset later (dark mornings keep it elevated) - Natural sleep window expands: 8-9 hours feels needed - Serotonin production lower (less light exposure)

The Yetish Study (2015): - Pre-industrial communities (no electricity) in Bolivia, Tanzania, Namibia - Winter sleep: 6.5-7.1 hours (but 1+ hour more TIME IN BED) - Summer sleep: 5.7-6.5 hours - Consistent finding: more dark = more rest, even if not all sleep

### Melatonin Seasonality

Winter melatonin profile: - Onset: 1-2 hours earlier than summer - Duration: 2-3 hours longer than summer - Amplitude: higher (more total melatonin produced) - Morning offset: later (explains difficulty waking in dark mornings)

Summer melatonin profile: - Onset: later (long evening light delays it) - Duration: shorter - Amplitude: lower - Morning offset: earlier (dawn light suppresses it quickly)

### Temperature Seasonality

Winter sleep advantages: - Cooler ambient temperature = easier sleep onset - Natural bedroom cooling supports deep sleep - Less nighttime sweating/waking

Winter sleep challenges: - Cold extremities can delay sleep onset (vasoconstriction) - Dry heated air → congestion → snoring/apnea worsening - Less exercise (cold/dark) → less sleep pressure

Summer sleep challenges: - Heat disrupts sleep architecture (less deep sleep above 75°F/24°C) - Night sweats increase awakening - Core temperature can't drop adequately in hot bedrooms - AC noise may help (white noise) or disrupt (temperature cycling)

Seasonal Sleep Disorders

### Seasonal Affective Disorder (SAD) and Sleep

Winter SAD (75-80% of SAD cases): - Hypersomnia (sleeping 9-12 hours but not feeling rested) - Delayed circadian phase (clock drifts later without morning light) - Increased carbohydrate craving (serotonin compensation) - Reduced deep sleep percentage (despite longer total sleep) - Lethargy misinterpreted as "laziness"

Summer SAD (rare, 10-15% of SAD): - Insomnia (can't sleep despite exhaustion) - Early waking from light - Agitation and anxiety - Weight loss

Treatment (winter SAD): - 10,000 lux light therapy, 30 min, within 30 min of waking - Dawn simulation alarm (gradual light before alarm) - Vitamin D supplementation (2000-5000 IU daily) - Fixed wake time (anchor the drifting clock) - Evening blue-light blocking (protect early melatonin onset)

### Insomnia Seasonal Patterns

• →Insomnia peaks in spring and fall (transitions are hardest)
• →Clock-change weeks (DST): insomnia complaints increase 30%
• →Summer insomnia: often heat or light-related (solvable)
• →Winter insomnia: often circadian drift or depression-related (needs treatment)

Seasonal Adjustment Protocol

### Spring Transition (Gaining Light)

Weeks 1-4 after spring forward: 1. Move bedtime 15 min earlier every 3-4 days 2. Increase morning light exposure (eat breakfast outside) 3. Add evening blue-light blocking (8 PM onward) 4. Exercise earlier in the day (shift energy expenditure forward) 5. Expect 2-3 weeks of adjustment (don't force it in 2 days)

### Summer Optimization

1. Blackout curtains (non-negotiable — 4:30 AM dawn will wake you) 2. Fixed wake time (don't let late sunsets drift your schedule) 3. Bedroom cooling (target 65-68°F/18-20°C) 4. Accept slightly less sleep (7-7.5 hours may be naturally sufficient) 5. Social jet lag guard (summer social pressure → late nights → Monday crash) 6. Hydrate before bed (summer dehydration disrupts sleep)

### Fall Transition (Losing Light)

Weeks 1-4 after fall back: 1. Gradually allow bedtime to drift 15-20 min earlier 2. Increase light exposure during the day (maximize available photons) 3. Start vitamin D supplementation (above latitude 37°) 4. Afternoon exercise (prevents energy drop from reduced light) 5. Monitor mood (SAD onset typically begins 2-4 weeks into reduced light)

### Winter Optimization

1. Dawn simulation alarm (wake with gradually increasing light) 2. Morning light box (10,000 lux, 20-30 min with breakfast) 3. Allow 20-30 min more sleep (don't fight the biology) 4. Consistent schedule (winter's dark mornings let the clock drift easily) 5. Evening routine earlier (use early melatonin onset as an asset) 6. Vitamin D (3000-5000 IU daily — supports serotonin→melatonin conversion) 7. Humidifier (40-50% humidity prevents dry air sleep disruption)

Chronotype × Season Interactions

Lions in winter: - Barely affected (dark mornings don't override strong morning drive) - May actually benefit from permission to sleep slightly later - Risk: SAD-like symptoms if they DON'T get morning light (needs light box)

Wolves in winter: - Most affected chronotype (dark mornings validate their late tendency) - Clock drifts VERY late without morning light intervention - Need aggressive morning light therapy - Actually feel best in summer (natural alignment with long days)

Bears in winter: - Moderate drift (30-45 min later, manageable) - Mood most affected (Bears are most SAD-susceptible) - Light therapy + exercise + social rhythm = prevention

Dolphins in winter: - Sleep anxiety increases (dark evenings trigger rumination) - Insomnia worsens without structure - Need strict routine + light therapy + possibly CBT-I refresher

Take our chronotype quiz to understand how YOUR biology interacts with seasonal light changes — then adjust your sleep strategy seasonally instead of fighting your body year-round.