The human Circadian Rhythm is the daily cycle that drives many physiologic activities in our body from digestion to hormone secretions to our ability to sleep.
The timing of human sleep is governed by the length of time since a person last slept and by their master body clock. The master body clock is located in a region of the brain called the suprachiasmatic nucleus (SCN). This master body clock controls the timing of many aspects of physiology, behavior and metabolism that show daily rhythms, including the sleep-wake cycles, body temperature, alertness and performance, metabolic rhythms and certain hormones which exhibit circadian variation. Outputs from the SCN control many endocrine rhythms including those of melatonin secretion by the pineal gland as well as the control of cortisol secretion via effects on the hypothalamus, the pituitary and the adrenal glands.
In most people, circadian rhythms match the 24-hour day by adjusting to environmental cues, the most powerful one being light. When a person's master body clock is misaligned to the 24 hour day-night cycle, it may result in a circadian rhythm disorder.