We all feel the ebb and flow of daily life, the daily rhythms that shape our days. The most basic daily rhythm we live by is the sleep-wake cycle, which (for most) is related to the cycle of the sun. It makes us feel sleepy as the evening hours wear on, and wakeful as the day begins. Sleep-wake and other daily patterns are part of our circadian rhythms, (circum means "around" and dies, "day") which are governed by the body's internal or biological clock, housed deep within the brain.
But research has been finding that the body's clock is responsible for more than just sleep and wakefulness. Other systems, like hunger, mental alertness, mood, stress, heart function and immunity also operate on a daily rhythm.