Hank Greene, an airline pilot, is having difficulty with the sleep-wakefulness portions of his circadian cycle. The pineal gland secretes the hormone , which influences this cycle.

Melatonin

Explanation:

Melatonin is the hormone that is produced by the pineal gland in response to the dark and light cycle. The pineal gland is turned on when there is darkness and turned off when there is light.When it's dark, the pineal gland becomes active and produces melatonin leading to an increase in the melatonin levels in the blood and a person starts to feel more sleepy and the levels of melatonin remain in the blood for about 12 hours after which they fall and the state of wakefulness occurs.The levels of melatonin fall sharply in the presence of bright light.Thus, the melatonin hormone is responsible for controlling the sleep-wake cycle.

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