People typically sleep at night -- thanks not only to the 9-to-5 workday, but also to their internal "clock." This clock is a small part of your brain that sits just above the nerves in the back of your eyes. Light and exercise "reset" the clock and can move it forward or backward. Issues related to this clock are called circadian rhythm disorders ("circa" means "about," and "dies" means "day"). They include jet lag, adjustments to shift work, delayed sleep phase syndrome (you fall asleep and wake up too late), and advanced sleep phase syndrome (you fall asleep and wake up too early).