Understanding how the human biological clock works is an essential step toward correcting sleep problems like insomnia and jet lag. New insights about the body's central pacemaker might also, someday, advance efforts to treat diseases influenced by the internal clock, including cancer, Alzheimer's disease and mood disorders.  Daniel Forger, from University of Michigan (USA), and colleagues have identified the signal that the brain sends to the rest of the body to control biological rhythms.  While it is generally accepted that the body's main time-keeper resides in a region of the central brain called the suprachiasmatic nuclei (SCN), the new research sheds light on the exact pattern of electrical pulses fired and overturns previous assumptions that held that the SCN  fired rapidly during the day and slow at night.  Through experiments conducted on mice, the team found that during the day, SCN cells expressing a key clock gene, per1, sustain an electrically excited state but do not fire. They fire for a brief period around dusk, then remain quiet throughout the night before releasing another burst of activity around dawn. This firing pattern is the signal, or code, the brain sends to the rest of the body so it can keep time.  Commenting that "[t]he old theory was that the cells in the SCN which contain the clock are firing fast during the day but slow at night. But now we've shown that the cells that actually contain the clock mechanism are silent during the day, when everybody thought they were firing fast," the team suggests that the findings "force us to completely reassess what we thought we knew about electrical activity in the brain's circadian clock."