The biological clock is a part of our body, which generates a number of biological responses regulated by cycles. 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.