pineal gland

https://sleeppsychiatrist.com/blog/pineal-gland-function-how-it-produces-melatonin-regulates-sleep/ 

Pineal Gland Function (Simple Explanation)

• The pineal gland is a small gland in the brain.

• Its main job is to make a hormone called melatonin.

• Melatonin helps control your sleep–wake cycle (when you feel sleepy and when you wake up).

• The pineal gland releases more melatonin when it is dark, so you feel sleepy at night.

• It releases less melatonin when it is light, so you feel awake during the day.

• The pineal gland also helps regulate biological rhythms such as body temperature and hormone cycles.

• It may also play a small role in mood, seasonal changes, and puberty timing.