Circadian rhythm is a cycle that operates on the twenty-four hour time span of one day. This is a cyclical process that governs all living organisms on Earth, humans included. One of the influencing factors that help control the rhythm is the sensing of daylight, the differentiation between day and night. Sometimes, this cycle is referred to as the "day-night" cycle, as a reference to the sensing of daylight.
For humans, most of us are active during the daytime hours. Our metabolic rates and body temperature rates are at their highest during these hours because of increased physical activity. Depending on the level of physical activity, metabolism and body temperature can fluctuate. Boys coming out of gym class, where they did a series of exercises and then ran for two miles, have their highest metabolic rates and body temperatures for the day.
When we go to bed at night, the metabolic processes and the body temperature slow down. The heart rate slows, the breathing slows, the body temperature dips slightly. This is when our metabolic rates and temperatures will be at their lowest. Our bodies are sleeping, on "pause", while the cells reenergize themselves for the next cycle, starting when we get up in the morning.