The eye isn't just an organ of sight. It contains special cells — ipRGCs — that respond not to images, but to the intensity and spectrum of light. These cells send a signal directly to the suprachiasmatic nucleus — the brain's biological clock. This is the main input for your circadian rhythm.
How to read the numbers: Target = the value we aim for · Risk = what to watch out for
Optimal for cortisol
Bright morning light helps set your internal clock and the natural morning rise in cortisol, and ordinary indoor lighting is far weaker than daylight outdoors. Step outside for a few minutes in the morning – even under cloudy skies there is more light than indoors.
Suppresses melatonin
The light-sensitive cells in the eye that steer your internal clock respond most strongly to blue light around 480 nm, which suppresses melatonin in the evening. For an hour or two before bed, dim your screens, switch on night mode and favour warm light.
Safe for sleep
Warm light with a colour temperature of around 2700 K or lower contains less blue, so in the evening it disrupts your body’s preparation for sleep less. Choose warm bulbs for the bedroom and living room, and in the evening prefer low, dimmed lamps.
Morning light — the most important biological ritual
The first 30–60 minutes after waking are critical. Exposure to bright light (ideally direct sunlight, at least 1,000 lux) sets your cortisol rhythm, dopamine and melatonin for the entire day. People who do this consistently fall asleep faster, sleep more deeply and have more energy during the day.
Typical indoor lighting: 100–300 lux. An overcast day outdoors: 1,000–10,000 lux. Direct sunlight: 30,000–100,000 lux. The eye needs enough light to set the circadian clock — and an office ceiling light isn't enough.
Evening blue light — the biological saboteur
Blue light (wavelength 480 nm) signals to the brain that it's daytime. Screens, LED lamps and smartphones emit it intensely, even at night. The result: melatonin suppressed by 50–80%, sleep delayed by 1–3 hours and disrupted sleep architecture.
