How the human body clock responds to light depends on its brightness and length of exposure

Exposure to 555-nm light was equally effective as 460-nm light at suppressing melatonin at the onset of nocturnal secretion.

• Cone photoreceptors may significantly contribute to the regulation of melatonin suppression and circadian phase resetting.
• The effectiveness of 555-nm light diminished over time compared to 460-nm light during exposure.
• At low irradiances, the influence of 555-nm light on phase resetting was greater than what could be accounted for by melanopsin activation alone.
• Melanopsin is suggested to be the primary photopigment for circadian responses during prolonged light exposure at high irradiances.
• Combining stimulation of both melanopsin and cone photoreceptors could enhance light therapy for sleep disorders.

Simplified