As the sun begins to set, Winston finds himself becoming more and more sleepy. What structure is sensitive to light and influences when to go to sleep and when to awaken?

Production of Melatonin

Explanation:

When the sun begins to set the light that enters the eyes is reduced. These uprachiasmatic nucleus tells Winston's body to produce the hormone Melatonin. This is the sleep hormone that prepares our body for sleep. The level of Melatonin will begin to rise two hours before we are scheduled for sleep. This is is the natural adaptation for sleep cycle and maintenance of the Circadian Rhythm.