The suprachiasmatic nucleus in the hypothalamus is the master circadian clock, receiving direct input from intrinsically photosensitive retinal ganglion cells containing melanopsin. These cells signal the SCN about the timing and intensity of environmental light, which the SCN uses to synchronise all peripheral clocks throughout the body: the liver clock, the gut clock, the adrenal clock, the skeletal muscle clock.
Going outside in the morning without sunglasses for 10 minutes does more for your sleep quality tonight than any sleep supplement.
Evening Light and Melatonin Suppression
A 2011 Journal of Clinical Endocrinology & Metabolism study found that room light before bedtime suppressed melatonin by 71% and shortened its duration to 5.7 hours versus 8 hours in dim light conditions. Amber or red-spectrum lighting (above 550 nm wavelength) does not activate melanopsin and does not suppress melatonin — making it the optimal choice for the 2 hours before sleep.