Glossary /
Blue Light
Short-wavelength visible light (roughly 450–490 nm) emitted by screens, LEDs, and the daytime sky.
What it is
Blue light is a region of the visible spectrum that specialized retinal cells — intrinsically photosensitive retinal ganglion cells, or ipRGCs — are most sensitive to. These cells do not contribute much to vision; their job is to signal “it is daytime” to the suprachiasmatic nucleus and suppress melatonin release.
Why it matters
Blue light is not a villain — it is information. Strong blue light in the morning is the cleanest possible circadian signal, which is why ten minutes outside before breakfast outperforms most “sleep hacks.” Strong blue light at night, however, tells your brain it is still midday and pushes melatonin onset back by hours. The fix is timing, not avoidance: bright daytime light, dim and warm light in the last few hours before bed, and screens kept off or dimmed in the bedroom.