The Cardiac rhtyms regulate changes in physical and mental characteristics that occur over the course of a day. The word circadian means "about a day." It comes from the Latin words "circa" (around) and "diem" (day).
The biological clock of our organism controls most circadian rhythms. This clock is in a region of the brain called the hypothalamus. But nevertheless, is not perfectly synchronized with the Earth, and that's why it delays two weeks with each passing year.
If in an experiment we were locked in a room isolated from sunlight and the outside world, our internal rhythm of sleep and wakefulness would adjust to a 25-hour cycle. That is, every 24 hours we would earn an hour. This could be verified, for example, in the summer of 1972, when the French Michel Siffre He lived in a cave in Texas for 7 months.
So? How do we do it in the real world? Basically, it is the light that stops this mismatch, which impacts our eyes, as it explains Henning Beck in his book To err is useful:
This allows us to understand the meaning of precision for neurobiology: while you delay the clock once a year to avoid being late for five minutes, your brain adjusts one hour daily. If this did not happen, our inner clock would accumulate a two-week error per year.
Since circadian rhythms are controlled by light, people who have some degree of blindness in both eyes have trouble sleeping.
Thus, for example, many people with total blindness have trouble sleeping for a lifetime because their eyes do not detect light. Melatonin supplements may improve the sleep of people with total blindness.
There are also other factors that calibrate the biological clock. These signals are called zeitgebers, a German word that means given time. The most important zeitgebers is light, both for plants and animals, but other zeitgebers include temperature, social activities, exercise and eating.
The longest cycle of 24.65 hours is the duration of the night-day cycle of the planet Mars. Therefore, it is presented as good news that our circadian rhythm could be adapted if we ever wanted to live on Mars