What is Pineal gland?
[[File:Illu_pituitary_pineal_glands.jpg|thumb|230px|Diagram of pituitary and pineal glands in the human brain]] [[File:Pineal_gland.png|thumb|200px|Pineal gland (in red)]] The '''pineal gland''' is a small [[endocrine gland]] in the middle of the [[brain]] of humans and other [[vertebrate]]s. It makes and releases the [[hormone]] named '''melatonin'''. There is almost no [[melatonin]] in the [[blood]] in the daytime. At the end of the day, melatonin in the blood allows [[sleep]]iness in humans and other animals which sleep at night. There is melatonin in the blood all night, while the person or animal is sleeping. In the [[morning]], there is less and less melatonin in the blood. When [[light]] enters the [[eye]]s, the melatonin of the blood goes away. Some people have [[circadian rhythm sleep disorder]]s which make them sleepy much earlier or much later than normal. The most common one is [[Delayed Sleep-Phase Disorder]] (DSPD, or Delayed Sleep-Phase Syndrome, DSPS). People with DSPD usually go to bed at 3 in the morning or later but they sleep normally for at least eight hours. == References == {{reflist}} * Eakin R.M 1973. ''The third eye''. Berkeley: University of California Press. * Guyton A.C. & Hall J.E. 2001. ''Textbook of medical physiology''. 10th ed, Saunders. {{biology-stub}} {{Endocrine system}} [[Category:Sleep]] [[Category:Glands]] [[Category:Hormones]]