A normal person sleeps 8 hours a night, 16 hours of awake. All our organs have a 24-hour life rhythm. However, this does not prove that we have a biological clock that regulates the body processes regardless of external events. Experiments carried out with volunteers who have gone underground have shown that a person has an internal, or "biological", clock, which, however, if not corrected by changing the light and darkness, go a little slower than expected: the 24-hour rhythm is imposed on us by the change of day and night and social signals.
Although we can measure very accurately about the time limits, they are not the same for us. The minute we spend in the dentist's chair seems much longer than the minute we spend in the movies or at our birthday party. How fast a week passes for an adult and how long it lasts for a small child! Psychologists can give you many interesting examples of how people perceive absolutely equal intervals of time in their memories or in reality.
Although time is equally fast around the world, a minute or a week may be different for us, depending on the situation in our lives. This circumstance was known in ancient times. But before the beginning of XX century there was a belief that time flows independently of the observer. So-called common sense suggested that for an astronaut flying in an interplanetary ship or in a strong gravitational field, the second lasts exactly the same as in our land. The great physicist Albert Einstein in his now confirmed theory of relativity has shown that at a very high speed and close to large masses of physics laws contradict common sense, or rather - our everyday experience. Thus, for example, the clock, which moves in an imaginary spacecraft, for an earth observer, goes slower than the same clock on Earth. If a spacecraft flies, say, at a speed of 99.9% of the speed of light a, it will take 22 seconds on Earth until the spacecraft will be only 1 second. In other words, the clock on the spacecraft for an earth observer will go 22 times slower than on Earth! This "relative" slowing down of time did not remain unknown for long, because it is manifested only at speeds close to the speed of light a, amounting to 300,000 km/s. Now this effect is easy to prove with the help of fast elementary particles, the duration of which for us is 80 microseconds, and for them only 1 microsecond. Their "clocks" are 80 times slower for an earth observer than on the Earth's surface.
There is no and cannot be an absolute time that is the same for all observers. Many scientists even believe that what we call time was not always the case. Clocks that move at almost the speed of light a are extremely slow. However, they, like all objects made up of matter, can never reach that speed limit. The "clock" of light particles, or quanta, that do not consist of matter and move exactly at the speed of light a, do not measure time at all. For light particles, unlike those consisting of matter of spacecraft, time does not exist. The concept of "time" makes sense only when there is a substance. But if the substance was formed at the moment of the Big Bang, then the time only arose. Thus, there is a time reference point, namely the Big Bang, for which there is no "before".
Nobody knows whether the Universe will ever stop expanding or always expand. Maybe someday it will start to fall back inside itself again. Matter will become denser in it and, finally, will form a black hole with infinite density of matter. But in this case the gravitational force would also become infinitely large and time would flow infinitely slowly, i.e. it would cease to exist. However, it may happen that the Universe will expand and become more discharged into it. Many physicists believe, however, that someday the matter will disintegrate into it, so that there will be only light particles for which there is no time. The decay of the last particle of matter would mean the end of time. This is difficult to imagine, but it is so: the time that seemed above all else in the world, above physics, turned out to be a secondary value, which has probably the beginning and the end. It came to us from the universe and will disappear with its disappearance.