Biological clock is an innate mechanism that controls the physiological activities of an organism which change on a daily, seasonal, yearly, or other regular cycle or we can also say it is an inherent timing mechanism in a living system that is inferred to exist in order to explain the timing or periodicity of various behaviors and physiological states and processes. Its control center is located in the hypothalamus, a gland located in the brain.
Also referred to as the circadian clock when talking about cyclic phenomena which are governed by the clock over a period of 24 hours. In particular, it regulates body temperature, and the cycle of sleeping and wakefulness with waking occurring in the morning and the need for sleep at night. There is also a circadian rhythm in certain hormones such as cortisol. When traveling with a time zone difference, the biological clock takes several days to readjust to changing light and schedules.