Simplify the following text for a Grade 12 native English-speaking person.
Original: The primary circadian `` clock '' in mammals is located in the suprachiasmatic nucleus ( or nuclei ) ( SCN ) , a pair of distinct groups of cells located in the hypothalamus .
Simplified: The primary body clock in humans and other mammals is located in the suprachiasmatic nucleus ( or nuclei ) ( SCN ) , a pair of groups of cells in the hypothalamus in the brain .