- Circadian rhythm refers to the natural, internal process that regulates the sleep–wake cycle and other biological rhythms within a roughly 24-hour period.
- Derived from the Latin words circa (about) and diem (day), circadian rhythms are driven by a central biological clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus in the brain. This clock synchronizes bodily functions with the external environment, particularly the light–dark cycle, ensuring that physiological processes occur at the most appropriate times of day.
- Circadian rhythms influence a wide range of biological functions, including sleep and alertness, hormone production, body temperature, digestion, blood pressure, and cell regeneration. One of the most well-known manifestations of the circadian rhythm is the sleep–wake cycle. During daylight, the SCN inhibits the release of melatonin, a hormone that promotes sleep. As darkness sets in, melatonin production increases, helping to induce drowsiness and signal that it is time to rest. This rhythmic pattern helps humans remain alert and active during the day and sleep at night.
- Light is the primary cue (or zeitgeber) that aligns the internal circadian clock with the environment. Photoreceptors in the retina detect light and send signals to the SCN, which in turn adjusts the body’s rhythms accordingly. However, non-light cues such as meal times, physical activity, and social interactions also play a role in modulating circadian rhythms. Disruption of these cues, especially light exposure at night—such as from screens or artificial lighting—can disturb the circadian system and negatively impact health.
- Circadian rhythms are not fixed; they change across the lifespan. For instance, infants have immature circadian systems, which develop within the first few months of life. Adolescents often experience a natural shift toward later sleep and wake times (a phenomenon known as “sleep phase delay”), while older adults tend to become sleepier earlier in the evening and wake up earlier in the morning. These shifts are influenced by both biological changes and lifestyle factors.
- Disruptions in circadian rhythms, known as circadian rhythm disorders, can arise due to jet lag, shift work, insomnia, or irregular sleep schedules. Chronic circadian misalignment has been linked to a variety of health problems, including sleep disorders, depression, metabolic syndrome, obesity, diabetes, cardiovascular disease, and even cancer. For instance, night shift workers, who often work against their biological clocks, are at higher risk for these conditions due to prolonged desynchronization between their internal rhythms and external demands.
- Understanding and maintaining healthy circadian rhythms is essential for overall well-being. Practices that support circadian health include exposure to natural sunlight in the morning, limiting screen time before bed, keeping a consistent sleep schedule, eating meals at regular times, and engaging in physical activity during the day. In clinical contexts, therapies like light therapy, melatonin supplementation, and chronotherapy (adjusting sleep times gradually) are used to treat circadian rhythm disorders.
