Scientifically, sleep is an altered state of brain activity that is significantly different from being awake. During sleep, our brain cells work slower but more intensively. This can be seen on the electroencephalogram: bioelectrical activity decreases in frequency, but increases in voltage. Consider the four stages of sleep and their characteristics. Breathing and heartbeat are regular, muscles are relaxed, body temperature decreases. We are less aware of external stimuli, and consciousness is slowly moving out of reality. The slightest noise is enough to interrupt this stage of sleep (without even realizing that you were sleeping at all). Approximately 10% of a night’s sleep passes in this stage. Some people tend to twitch during this period of sleep (for example, fingers or limbs). Stage 1 usually lasts from 13-17 minutes. This stage is characterized by a deeper relaxation of the muscles and sleep. Physical perception slows down significantly, eyes do not move. Bioelectrical activity in the brain occurs at a lower frequency compared to wakefulness. The second stage accounts for about half of the time spent on sleep. The first and second stages are known as light sleep phases and together they last about 20-30 minutes. During sleep, we return to the second stage several times. We reach the deepest phase of sleep at about 30 minutes, stage 3, and at 45 minutes, the last stage 4. Our body is completely relaxed. We are completely disconnected from what is happening around the reality. Significant noise or even shaking is needed to awaken from these stages. Waking up a person who is in the 4th stage is almost impossible – it is akin to trying to wake up a hibernating animal. These two stages make up 20% of our sleep, but their share decreases with age. Each of the stages of sleep serves a specific purpose for the body. The main function of all phases is the regenerative effect on various processes in the body.