Sleep is a physiological, changeable state of consciousness and mind characterized by reduced activity and response to external sensory stimuli. There are two phases of sleep: the REM and NREM phases of sleep. Together they form one sleep cycle that lasts about 90 minutes and is repeated up to six times during sleep. A specific phenomenon occurs during sleep, the role and appearance of which many scientists want to explain even today, and that phenomenon are dreams. Dreams are a series of images, ideas, emotions and feelings that involuntarily occur in the mind during sleep. Most dreams take place in the REM phase of sleep and such dreams are described as vivid and often bizarre as opposed to dreams in the NREM phase which are harder to remember after waking up and which are more realistic. Evolutionarily speaking, the development of sleep has been gradual and has become more complex. Reptiles were the first species to develop a form of sleep that we can call primitive sleep. The appearance of birds and mammals and their ability to maintain a constant body temperature led to the development of sleep as we know it today, which consists of the NREM and REM phases of sleep. Sleep almost always begins with the NREM phase which is divided into three subphases and flows from light sleep to deep sleep which ends the NREM phase of sleep. Looking at the EEG record, we can see that brain activity gradually decreases from the alpha and beta brain waves present in the waking state to the slowest theta brain waves present in deep sleep. At the end of the NREM phase, the REM phase of sleep begins, in which brain activity increases and alpha and beta brain waves can be seen on the EEG. Physiological changes that occur during sleep affect almost all organic systems. Changes affecting the autonomic nervous system from the increase of the parasympathetic activity to the decrease of the sympathetic activity affect the whole organism and reduce its activity. There is no single centre in the brain that is responsible for sleep regulation and the formation of dreams - sleep is a process that occurs through the interaction of numerous brain structures that use neurotransmitters and neuromodulators to correct their activity and create the experience of dreams.