Question

The hormone , which is secreted by the pineal gland, influences the sleep–wakefulness cycles.

Answer 1

Melatonin is the answer

Answer 2

The hormone described above is the hormone Melatonin.
Melatonin is a hormone secreted by the pineal gland in the brain, which helps regulate other hormones and maintains the body's circadian rhythm. This rhythm is an internal 24 hour clock that plays a critical role when we fall asleep and when awake. When it is dark the body releases more melatonin.