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. 

Read more about this at this [previous answer][1].


  [1]: http://biology.stackexchange.com/questions/7667/what-causes-humans-to-be-sleepy/7672#7672