The release of the hormone melatonin is responsible for the feeling of sleepiness. It is released by the pineal gland and production starts when the light fades, as it's production is inhibited by light stimulation of the retina, the onset of the production is called dim-light melatonin onset.
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This study on blind people will show you that the light entering the eyes is the (main at least) regulator of melatonin rhythms. They took blind people, 19 of whom could perceive light and 30 who could not, and showed that 14 of 19 light perceivers had normal melatonin rhythms while 23 of the light-blind people had abnormal or unclassifiable rhythms.
