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Blue Light

Sleep The strongest negative impact. Suppresses melatonin production — even 2 hours of evening exposure can shift the circadian rhythm by 1.5–3 hours. The brain reads it as midday sunlight.

Mood & Mental Health Increases alertness and cortisol — useful in the morning, harmful in the evening. Chronic exposure (screens) is linked to anxiety, irritability, and depression.

Physical Health

• Damages retinal cells with prolonged exposure
• Disrupted sleep triggers inflammatory processes and weakens immunity
• Associated with higher risk of obesity and cardiovascular disease through hormonal dysregulation

Red Light

Sleep Minimally affects melatonin — ideal evening lighting. The brain does not read it as an alert signal.

Mood & Mental Health Low intensity: grounding and calming. High intensity (e.g. an all-red room): may increase arousal or aggression — context dependent.

Physical Health (Red Light Therapy) Wavelengths 630–850 nm have solid research behind them:

• Stimulates mitochondria (ATP production)
• Anti-inflammatory effects
• Tissue healing, muscle recovery
• Improved sleep quality when used in the evening

Green Light

Sleep Moderately suppresses melatonin — significantly less than blue. It sits in the middle of the spectrum.

Mood & Mental Health The most natural wavelength for the human eye — we are evolutionarily most sensitive to it. Associated with calm, balance, and stress reduction. Studies show reduction of migraines and chronic pain (narrow green spectral band ~530 nm).

Physical Health

• Harvard research: green light reduces pain intensity in migraine sufferers
• Less visually fatiguing than blue light
• Positive mood effects through nature association (biophilia)

Practical Summary

Key principle: Light spectrum is a temporal signal for the brain, not merely a matter of comfort. Chronobiology considers it one of the strongest synchronisers of the circadian rhythm — more potent than food or exercise.