Circadian rhythms
A circadian rhythm is the roughly 24-hour cycle that controls and regulates body's sleep cycles. Most commonly known as the body's internal clock, it is responsible for keeping the body in sync with the day and night cycle that the environment provides.
the body's internal clock
A group of nerve cells in the brain called the suprachiasmatic nucleus, or SCN, is responsible for the body's abilitity to determine sleep patterns. This group of nerve cells is located above the optic nerve, which is why circadian rhythms respond primarily to the amount of light in the organism's environment.
In the event that the retina is exposed to light, the hypothalamus signals the pineal gland to decrease melatonin production, a hormone that regulates sleep and wake cycles. This low melatonin production is the reason why you naturally wake up in the morning. During the nighttime, melatonin production increases, encouraging the organism to go to sleep.
During jet lag, the body's circadian rhythm is disrupted. However, due to the body's ability to quickly adjust to new environments, the body's internal clock will adapt to the new environment gradually over time.
hibernation
For animals that cannot survive the winter without migrating to warmer environments, they enter a state called hibernation. Hibernation is a state of inactivity that causes their breathing and heart rates to slow, and their body temperature to drop in order to use less energy. Their circadian rhythms, as a result, are bypassed in order to accomodate for the longer sleep cycles of the organism.
