The pineal gland.
D000818:Animals; D001683:Biological Clocks; D006066:Gonads / Q000502:physiology*; D006801:Humans; D008032:Limbic System / Q000502:physiology*; D010507:Periodicity; D010870:Pineal Gland / Q000502:physiology*; D011627:Puberty / Q000502:physiology; D012725:Sexual Behavior