Normal sleep is composed of two different sleep phases, non-rapid eye movement sleep (NREM) (or slow wave sleep) and rapid eye movement sleep (REM) (or fast wave sleep). During the whole sleep process, NREM sleep and REM sleep alternate, and the alternation is called one sleep cycle, from the beginning of the slow wave sleep period (accounting for about 90 minutes) to the fast wave sleep period (accounting for 10~30 minutes). NREM sleep is divided into stages 1 to 4, of which stages 3 and 4 are usually referred to as deep sleep, that is, effective sleep, which plays an important role in eliminating fatigue and restoring energy, and accounts for only 15% of the total sleep time each day and night. For example, a normal adult sleeps for 8 hours at night, at the beginning of the night, it first enters NREM sleep period, and quickly moves from stage 1 to stages 2, 3 and 4 in sequence, lasting about 90 minutes, which means that it usually enters the first deep sleep after 60 minutes of sleep. Then a REM sleep (lasting 10-30 minutes) occurs. The stages in each cycle are not necessarily complete, but they all start with stage 1. From about 4:00 am onwards, as melatonin production in the body gradually decreases, the depth of sleep in each cycle becomes shallower and no longer reaches stages 3 or 4. As a result, it is easier to wake up in the morning. Thus, from a physiological point of view, it is easy to get deep sleep between 24:00 and 4:00 at night. The slow wave sleep period is an important time for us to eliminate fatigue, recover energy and get deep sleep, while the fast wave sleep period is related to dreaming. People who wake up in the morning in the fast wave sleep period will have a memory of dreaming; while those who wake up in the slow wave sleep period may have no memory of dreaming. Therefore, a person who has a memory of dreaming every day does not imply a disease; only if you wake up the next day without a sense of physical recovery and still feel sleepy, you should pay attention to whether there is a problem of insomnia.