Pineal gland
This pea-sized gland is associated with our biological clock. It secretes melatonin which regulates circadian rhythm. The animation below shows the location of pineal gland in the brain.

Melatonin is secreted by the pineal gland during darkness. It has certain hypnotic properties. Studies suggest that melatonin helps in recovery of sleep in insomniac patients with Angelman syndrome. The characteristics of Angelman syndrome include severe intellectual disability, motor impairment, seizures and dysmorphic facial features. Their behavioural profile includes excessive laughter, hyperactivity, noncompliance, speech impairment, and insomnia followed by frequent waking-up during night. In such cases, melatonin has proved to significantly advance sleep onset, decrease sleep latency and increase total sleep time.