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The observation of a circadian or diurnal process in humans is mentioned in Chinese medical texts dated to around the 13th century, including the Noon and Midnight Manual and the Mnemonic Rhyme to Aid in the Selection of Acu-points According to the Diurnal Cycle, the Day of the Month and the Season of the Year. [7]
A diurnal cycle (or diel cycle) is any pattern that recurs every 24 hours as a result of one full rotation of the planet Earth around its axis. [1] Earth's rotation causes surface temperature fluctuations throughout the day and night, as well as weather changes throughout the year. The diurnal cycle depends mainly on incoming solar radiation. [2]
A circadian rhythm is an entrainable, endogenous, biological activity that has a period of roughly twenty-four hours. This internal time-keeping mechanism is centralized in the suprachiasmatic nucleus (SCN) of humans, and allows for the internal physiological mechanisms underlying sleep and alertness to become synchronized to external environmental cues, like the light-dark cycle. [4]
Within each cycle, the time period during which the process is more active is called the acrophase. [4] When the process is less active, the cycle is in its bathyphase or trough phase. The particular moment of highest activity is the peak or maximum; the lowest point is the nadir.
A chronotype is the behavioral manifestation of an underlying circadian rhythm's myriad of physical processes. A person's chronotype is the propensity for the individual to sleep at a particular time during a 24-hour period.
Sleep inversion may be a symptom of elevated blood ammonia levels [2] and is often an early symptom of hepatic encephalopathy. [3] Sleep inversion is a feature of African trypanosomiasis, after which the disease takes its common name, "African sleeping sickness"; [4] sleep-wake cycle disturbances are the most common indication that the disease has reached the stage where infection spreads into ...
Chronodisruption, in the form of shift work, increases the risk of breast cancer in women by about 50%. [13] The risk of developing other forms of cancers, such as prostate cancer in men and colorectal cancer in women, may also increase with chronodisruption; studies in this area have shown modest, but statistically significant, associations. [13]
For example, seasonal cathemerality has been described for the mongoose lemur (Eulemur mongoz) as activity that shifts from being predominantly diurnal to being predominantly nocturnal over a yearly cycle, but the common brown lemurs (Eulemur fulvus) have been observed as seasonally shifting from diurnal activity to cathemerality. [5]