The circadian rhythm is a natural, endogenous process that regulates the sleep-wake cycle and occurs approximately once every 24 hours. It is a natural oscillation that occurs approximately every 24 hours. It is controlled by an internal “biological clock” in the brain, notably the hypothalamic suprachiasmatic nucleus. The term “circadian” is derived from the Latin words “circa” (meaning “around”) and “diem” (meaning “day”), and refers to the rhythm’s approximately 24-hour period.
Circadian rhythms can refer to any process that begins within an organism (i.e., endogenous) and responds to (is entrained by) the environment. External stimuli, the most powerful of which is light, influence the circadian rhythm. Light, particularly natural sunlight, aids in the regulation of the internal clock. Other factors, such as meal timing, social interactions, and physical activity, can also influence the circadian rhythm.
Circadian rhythms are governed by a circadian clock, whose principal role is to rhythmically coordinate biological activities so that they occur at the optimal time to maximize an individual’s fitness. Circadian rhythms have been widely seen in mammals, plants, fungi, and cyanobacteria, and there is evidence that each of these kingdoms of life developed independently.
Key features of the circadian rhythm include:
- Sleep-Wake Cycle: The sleep-wake cycle is the most well-known feature of the circadian rhythm. The internal clock of the body impacts when we feel alert and awake throughout the day and tired at night.
- Body Temperature: Circadian rhythms can influence body temperature, with a trough in the early morning hours and a high in the late afternoon or early evening.
- Hormone Production: Certain hormones, such as melatonin (which helps regulate sleep), cortisol (which is connected to stress and wakefulness), and growth hormone, have a circadian pattern of release.
- Cognitive and Physical Performance: Cognitive functions, alertness, and physical performance can also show variations throughout the day, influenced by the circadian rhythm.
- Metabolism: The circadian rhythm can affect metabolic processes, including digestion and energy expenditure.
The phrase circadian is derived from the Latin words circa, which means “approximately,” and dies, which means “day.” Processes with 24-hour cycles are referred to as diurnal rhythms; diurnal rhythms should not be referred to as circadian rhythms until they can be proven to be endogenous rather than environmental.
Circadian rhythm disruptions, such as those induced by shift work, irregular sleep habits, or jet lag, can result in a variety of health problems, including sleep difficulties, mood swings, and reduced cognitive function. A consistent sleep schedule, exposure to natural light during the day, and other lifestyle factors can all contribute to a healthy circadian rhythm.
