In humans, the Suprachiasmatic Nucleus, abbreviated SCN, is a region of the brain, in the anterior hypothalamus that is the master circadian pacemaker of the body. The SCN interprets light signals and uses them to set the body's internal clock. In the absence of light or other circadian signals, the SCN will maintain a circadian period of about 24.6 hours. However, the SCN is constantly adjusting the clock based on sensory input, which is why humans are able to live on a 24 hour schedule.
Kryger, M., Roth, T., & Dement, W. (Ed.) Principles and Practice of Sleep Medicine (4th Ed.). Philadelphia: Elsevier, 2005. ISBN: 0-7216-0797-7