Pineal gland

The pineal gland is a feature of Human anatomy as well as most animals. It creates a hormone to maintain sleep cycles.

History
Leonard McCoy transplanted a pineal gland aboard the USS Enterprise (NCC-1701) in 2273. 

In 2379, Doctors Beverly Crusher and Tropp researched a treatment on Tognetti, a member of the USS Enterprise (NCC-1701-E) crew. The Bandi chief had been exposed to a gas which tripled serotonin levels, forcing his body to adjust pineal hormones which resulted in an accelerated life cycle.