Biological Rhythms
The body as a whole or its components is in a dynamic state where the physiological process repeats itself with more or less constant time intervals. The phenomenon is defined as a ‘biological rhythm’ which is influenced by a ‘biological clock’.
The variations in rhythms may be as a consequence of interactions with external physical cycles, such as daylength, temperature, humidity and atmospheric pressure. Hence, the biological rhythms may be internal (endogenous) which is regulated by the internal biological clock such as body temperature cycle and external (exogenous) factors which is governed by the coordination of internal cycles with external stimuli such as sleep/wakefulness and day/night. These stimuli are known as zeitgebers which include environmental time signals such as sunlight, food, noise and social interaction. The zeitgebers are supportive in resetting of the biological clock to a 24-h day.The animals accomplish behavioural patterns in a day at a constant time interval which are triggered by habit and daily fluctuations in environmental conditions. However, some physiological events are having their own individual biological clocks such as heart rate, body temperature and metabolism. These clocks get out of phase with each other besides the time and adjusted to 24-h day and night cycles and are termed as ‘circadian rhythms’ (Latin words circa ‘about’ and diem ‘day’). Circadian rhythms are elucidated as innate self-sustaining oscillations which are brought into a specific rhythm by the environmental factors especially light and temperature. A cycle or rhythm is established during an event occurs regularly through a peak and a trough over a particular period of time. The biological rhythms are classified into three types as per the variations in the duration of time period that is the time interval separating one peak or trough from the next in a repeating cycle. A circadian rhythm that is synchronized with the day or night cycle is called as ‘diurnal rhythms’. Further, biological rhythms such as feeding cycles which have shorter rhythm (shorter than a circadian rhythm) are termed as ‘ultradian rhythms’. The biological rhythms with a cycle of more than 24 h are referred as ‘infradian rhythms’ such as estrus cycle in animals.
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