Light Exposure Science

Why Light Is So Powerful

Light is the primary zeitgeber (time-giver) for your circadian system. Your eyes contain specialized photoreceptors called intrinsically photosensitive retinal ganglion cells (ipRGCs) that detect light and send signals directly to the suprachiasmatic nucleus (SCN)—your brain's master clock.

Unlike other circadian influences (meals, exercise, social cues), light has the most direct and powerful effect on shifting your internal clock. This makes strategic light exposure the cornerstone of any effective jet lag prevention strategy.

How Light Shifts Your Circadian Rhythm

The Phase Response Curve

The effect of light on your circadian rhythm depends critically on when you're exposed to it. This relationship is described by the phase response curve:

• Light in the Late Night/Early Morning: Delays your rhythm (makes you want to sleep later)
• Light in the Late Evening: Advances your rhythm (makes you want to sleep earlier)
• Light in the Middle of Your Day: Has little effect on phase shifting

Understanding Your Core Body Temperature Minimum

The timing of light exposure is usually referenced to your core body temperature minimum (CBTmin), which typically occurs about 2-3 hours before your natural wake time.

• Light before CBTmin: Delays your clock (phase delay)
• Light after CBTmin: Advances your clock (phase advance)

Example: If you normally wake at 7 AM, your CBTmin is around 4-5 AM.

Light Strategies for Eastward Travel

When traveling east, you need to advance your circadian phase (shift earlier).

Before Departure

• Seek bright light in the early morning (as early as comfortable, ideally right after waking)
• Avoid bright light in the evening (use dim lights, avoid screens, wear amber-tinted glasses if needed)
• Gradually shift your wake time earlier by 30-60 minutes per day
• Consider using a light therapy lamp (10,000 lux) for 20-30 minutes after waking

After Arrival

• Get bright outdoor light as soon as possible in the destination morning
• Spend 30-60 minutes outside in the morning (even if it's cloudy)
• Avoid bright light in the late evening—use dim lighting after dinner
• If you arrive very early morning, avoid bright light until closer to local sunrise

Light Strategies for Westward Travel

When traveling west, you need to delay your circadian phase (shift later).

Before Departure

• Seek bright light in the evening (stay up later with bright lighting)
• Avoid bright light in the early morning (sleep in if possible, use dim lights, wear sunglasses if you must be up early)
• Gradually shift your bedtime later by 30-60 minutes per day
• Engage in bright light activities in the evening (outdoor activities, bright indoor spaces)

After Arrival

• Get bright light in the destination late afternoon and evening
• Stay active outdoors until closer to destination bedtime
• Avoid bright light in the early morning of the first few days
• Sleep in or use dim lights if you wake up early

Types of Light: What Works Best

Natural Sunlight

Best Option: Outdoor daylight is ideal

• Intensity: 10,000-100,000+ lux (even cloudy days provide 1,000-10,000 lux)
• Full spectrum: Contains all wavelengths needed for circadian entrainment
• Additional benefits: Physical activity, fresh air, temperature changes
• Duration: 30-60 minutes provides strong circadian signal

Light Therapy Lamps

Good Alternative: Especially useful before departure or in winter

• Intensity: Look for 10,000 lux at recommended distance
• Blue-enriched white light: Most effective for circadian shifting
• Duration: Typically 20-30 minutes
• Positioning: Place to the side at eye level, not directly in front
• Safety: UV-filtered; consult doctor if you have eye conditions

Indoor Lighting

Less Effective but Better Than Nothing:

• Typical indoor lighting: 100-500 lux (much dimmer than needed)
• Improvement strategies: Open blinds, turn on multiple lights, sit near windows
• Ceiling lights > desk lamps: Light from above is more effective

Blue Light and Screens

Mixed Effects:

• Blue wavelengths (460-480 nm): Most effective for circadian phase shifting
• Screens: Tablets and phones emit blue light but at lower intensity than needed for strong circadian effects
• Evening use: Can interfere with sleep preparation; use blue-blocking mode or glasses before bed
• Morning use: Can provide a mild circadian stimulus, though not as effective as bright light

Light Avoidance Is Equally Important

Getting light at the wrong time can slow your adjustment or shift you in the opposite direction. Light avoidance strategies:

Practical Avoidance Techniques

• Sunglasses: Wear wraparound sunglasses when you need to be outside during "avoid" times
• Dim indoor lighting: Use lamps instead of overhead lights; lower brightness
• Blue-blocking glasses: Amber-tinted glasses filter blue wavelengths in the evening
• Blackout curtains: Essential for sleeping during destination daytime
• Night mode on devices: Enable warm color filters on phones and computers
• Red lights: Red wavelengths have minimal circadian effect

Practical Light Exposure Guidelines

Intensity Matters

Minimum effective light levels for circadian shifting:

• Threshold: ~1,000 lux begins to have an effect
• Optimal: 10,000 lux provides strong signal
• Dose-response: Brighter is generally better (up to a point)

Duration Matters

• Minimum: 15-20 minutes of bright light
• Optimal: 30-90 minutes for maximum effect
• Continuous vs. intermittent: Continuous exposure is more effective, but intermittent can work

Timing Is Critical

Light at the right time can shift your clock by 1-3 hours per day. Light at the wrong time can shift you in the opposite direction or slow your adjustment significantly.

Special Considerations

Very Long Flights (8+ Time Zones)

For extreme time zone changes, you might shift in the "other direction." For example, instead of advancing 10 hours for a Tokyo to New York flight, it might be easier to delay 14 hours (going "the long way around the clock").

Crossing the International Date Line

Flights crossing the date line (e.g., Los Angeles to Tokyo) can be particularly disorienting. The same light exposure principles apply—focus on the direction of shift needed rather than the geographic direction.

Layovers and Multi-Stop Trips

For trips with layovers in intermediate time zones, decide whether to adjust partially to each zone or try to maintain either home time or final destination time. The best strategy depends on layover duration and your final destination.

Common Mistakes to Avoid

• Getting morning light when traveling east without considering your CBTmin: If you wake up too early (before your CBTmin has shifted), morning light will delay rather than advance your rhythm
• Staying indoors all day: Indoor lighting is usually insufficient for circadian shifting
• Not avoiding light at critical times: Light avoidance is as important as light seeking
• Using sunglasses all day: Blocking light when you should be seeking it
• Inconsistent timing: Random light exposure is less effective than strategic, timed exposure

Key Takeaway: Light is your most powerful tool for shifting your circadian rhythm. Strategic exposure and avoidance, timed according to your travel direction, can dramatically reduce jet lag symptoms and speed up adjustment.