Pineal, PIHN ee uhl, gland, also called pineal body, is a tiny organ in the brain of human beings and most other vertebrates (animals with a backbone). Scientists are uncertain of the function of the pineal gland in human beings. They believe it plays a role in a variety of important body functions, including certain reproductive processes. In most other vertebrates, the pineal gland helps regulate certain daily body cycles, called circadian rhythm, and seasonal body cycles.