The pineal gland produces and secretes the hormone melatonin. This hormone is involved in regulating sleep cycles, circadian rhythms and seasonal metabolic and reproductive changes.
Role of melatonin in sleep
Melatonin is sometimes called the sleep hormone because it signals the brain to induce a feeling of sleepiness. It is synthesized from the neurotransmitter serotonin in response to environmental light levels, increasing when it is dark — to prepare an organism for sleep — and decreasing when it is light — in preparation for waking.
How does the pineal gland know whether it is light or dark?
When light enters the eyes, a signal is transmitted to the hypothalamus, the gland responsible for regulating circadian rhythm. From there the signal passes to the spinal cord and then through the superior cervical ganglia to the pineal gland. In response, the pineal gland stops synthesizing and releasing melatonin. When the light signal stops, melatonin synthesis and secretion resumes.