Circadian rhythms are the cyclical variations that mark the ebb and flow of daily life.
These include waking and sleeping, the rise and fall of blood pressure and heart rate, and other factors such as gastrointestinal, renal, and immune system activity. Normally the internal body clock aligns its timing to the external environment through exposure to sunlight. If the clock is stimulated by light before it expects the sun to have risen, it will reset itself to an earlier time. If stimulated by light after the time it expects the sun to have set, the body clock will reset itself to a later time. Sleep phase disorders occur when a person's internal biological clock is normally delayed or advanced from the external environment.