How evening light holds back melatonin
Melatonin is the hormone that signals biological night, normally rising in the evening as darkness falls. Light reaching the eye suppresses its release through a dedicated pathway: specialized retinal cells containing the pigment melanopsin, most responsive to blue wavelengths, report light levels to the brain's master clock, which in turn tells the pineal gland to hold off on melatonin.
Sensitivity is high enough that even modest indoor light in the evening can blunt the rise, and individuals differ considerably in how strongly they respond.
Why screens and bright rooms delay sleep
When melatonin is suppressed by evening light, the internal signal that it is time to sleep is weakened and delayed, which can push back the time sleep comes easily. Bright phone, tablet, and computer screens viewed close to the eyes contribute to this effect, particularly late at night.
Exact figures depend on light intensity, wavelength, duration, and timing, so single numbers are approximate. This is general educational information about sleep physiology, not medical advice.