Sleep is a natural phenomenon that is widespread among all species in nature, and we have been studying it at all levels. Research on sleep has focused on three main areas: What is sleep? What are the mechanisms of sleep? What is the function of sleep? To answer these questions well, it is necessary to draw on the latest research methods in basic and applied neuroscience. In addition, the study of sleep also contributes to the understanding of consciousness, which changes in response to sleep-induced changes in brain activity. For decades, it has been believed that brain activity is greatly diminished or even disappears when a person is asleep. Subjective sensations such as feeling a loss of consciousness in the sleep state and the disappearance of memories of mental activity seem to support these views. Even two great scientists, like Charles Sherrington and Ivan Pavlov, also support this idea. However, in the 1950s and 1960s, scientists discovered the phenomenon of alternating periods of rapid eye movement (REM) sleep and slow eye movement (non-REM, NREM) sleep, and the above-mentioned view of sleep was overturned. Since REM sleep periods are associated with hallucinatory dreaming, this proved that the brain is indeed highly active during sleep periods. Soon after the discovery of this phenomenon, it was noticed that if the brain was “activated” during REM sleep, the sensory input and motor output would be interrupted at the same time, and some researchers figuratively referred to this phenomenon as “off-line” (‘off-line’). During the 90-minute REM sleep period, the brain is in a high state of arousal. The REM sleep period accounts for 20 percent of the entire sleep process. This fact disproves the previously widely accepted view that sleep is formed due to the cessation of brain activity. Other evidence has confirmed that the brain stays active during sleep. The earliest cerebral blood flow (CBF) studies conducted by Seymour S. Kety and, after him, Sokolov, showed that cerebral blood flow to the brain was reduced by only 20% during the sleep state. During the sleep state, it was found that as many brain neurons with reduced neuronal cell activity as there were brain neurons with an almost equal number of increased neural impulse frequency (firing rate). Given the correlation between blood flow and neuronal cell activity, it is not surprising that this phenomenon occurs. Even during NREM sleep, when consciousness is completely lost, the brain remains active to a certain extent. In early work done in laboratories conducting sleep research, researchers conducted a descriptive study of sleep using techniques such as brain imaging and quantitative electroencephalography (EEG), and obtained fruitful results. The imaging techniques revealed that the localized regions of the brain were very different between the two EEG activation states, i.e., REM sleep and wakefulness, and that the brain activity in both states was different from that in NREM sleep, when the EEG showed high voltage slow waves instead of low voltage fast waves. Quantitative EEG analysis also revealed different electrical activity between brain regions. These new data suggest that the brain is relatively quiescent during NREM sleep, when the brain waves consist mainly of spindle waves and high voltage slow waves. However, it is important to emphasize that this overall state of brain quiescence is only relative. Although consciousness is sluggish at this time, the brain is still about 80% active and has fine information processing capabilities. Therefore, the EEG sleep spindle and slow waves represent changes in the excitability of the electrical activity of the cortical and thalamic areas of the brain, and these waves cannot be considered subjectively as “noise”, they are also a signal that the brain needs to perform certain functions. 1. Mechanisms and functions of sleep In general, all research findings support two views. The first view is that sleep is an active process controlled by the brain, rather than a passive result of the non-awake state; the other view is that sleep should be seen as a reorganization of neuronal activity in the brain rather than a cessation of activity. According to the first view, it is clear that although mammalian sleep occurs at resting times in the circadian rhythm, it is in fact still the result of active control by the hypothalamus (hypothalamus) and the brainstem. It is on this premise that Saper’s recent description of the sleep switch (see “Studies on the mechanisms of hypothalamic regulation of human sleep and circadian rhythms” for details) has gained wide acceptance and acclaim. Almost all mammals that have been studied show NREM-REM alternation during sleep, suggesting that sleep is not only a universal mechanism, but also that its role is by no means as simple as saving energy as one might imagine. The finding that sleep has a continuous, fine-grained regulatory mechanism further suggests that sleep must have additional functions. For example, the recent discovery that sleep has a role in controlling the body’s energy stability as well as enhancing the ability to learn. This also recalls Stickgold’s idea that sleep enhances procedural learning (see “Sleep-dependent memory consolidation processes” for details). Now, there is a renewed enthusiasm in the research community for the field of sleep and learning ability. This is mainly due to the fact that researchers have observed in terrestrial mammals that sleep can help these animals to reinforce learned motor skills. There are some animals that are still able to learn even when very sleep deprived, but this does not mean that the link that has been found between sleep and improved learning ability does not exist. The general phenomenon is that most animals would use sleep to help with learning if they could. To elucidate the question of how sleep helps (or does not help) narrative memory, there is a more difficult difficulty to be solved, which requires more elaborate experimental design and hard work to do. The next issue to consider is that if memory refers to the conscious recall and organization of what has been learned, then we can discuss the question of how sleep helps memory. According to the above definition, memory depends on learning, but is in no way equivalent to it. In-depth studies of sleep differences between species and between age groups of the same species have shown that sleep has multiple functions. These functions vary among species, even if a particular function is missing in one species. The fact that the same functions may be achieved at different stages of the waking state does not mean that sleep does not work, but rather that species with relatively little sleep must have other channels to satisfy this requirement, and this “plasticity” is clearly multifunctional. There are many functions and mechanisms in the brain that are related to sleep. It was this view that inspired and supported Siegel’s phylogenetic studies (see “Mammalian Sleep Function Explored” for details). Sleep phylogeny has been studied for a long time, and it soon became apparent that sleep usually occurs only in theropods with relatively large brain size. Allison and Cichetti’s study showed that the number of sleeps, the depth of sleep, and the distribution of sleep times varied among species in order to adapt to different ecological niche. The general rule is that lions like large carnivores like open-air habitat, in the non-foraging and mating time, their sleep time are longer, sleep is also deeper; in contrast, rabbits like small herbivores like burrowing, their sleep time is shorter, because they need more time to forage and mating. At the same time, they must also remain awake to prevent predation. Therefore, this study concludes that if animals can sleep, they will sleep. Studies of human sleep have also shown that sleep is highly variable, as researchers have identified many very complex types of sleep disorders. If sleep is considered to be an actively regulated process, it is possible to understand why some people sleep so little, while others sleep so much, and still others sleep at different times than the average person. Mahowald and Schenck have written a review of these sleep disorders, noting that many of the conditions that separate sleep and wakefulness states can be seriously disturbing (see “Advances in Sleep Disorders Research” for details). They noted that many sleep-wake state dissociations can be a serious problem (see “Advances in Sleep Disorders Research”). What are the implications of the latest technological advances in sleep for theories related to dreaming? Contrary to Freud’s view of dreams as a response of memory to daytime experiences, the latest data suggest that dreams are laced with a large number of memory fragments that are pre-represented in dreams up to 6 days in advance. However, the finding of awaiting systematic replication suggests that any theory of the cause of dreams should be accepted in a tolerant and non-judgmental manner, even if it has no basis. Nielsen’s research is aimed at exploring the secrets behind these phenomena (see “What is the source of dream memory?” for details). 2. Sleep and consciousness 2. Sleep and consciousness Probably one of the most important concerns of modern sleep science, the mystery of the nature of consciousness, is not mentioned in this paper. Momentary responses confirm that consciousness is state-dependent. For centuries, we have misunderstood the relationship between sleep and the brain because we have made the erroneous judgment that consciousness ceases during sleep and becomes active again only after waking. However, occasional dreams disprove the above theory, but numerous great scientists, including Sigmund Freud, have also incorrectly assumed that dreams only occur during waking. Although it is true that no consciousness exists during NREM sleep, it still changes along with brain activity just after falling asleep (when there is dream-like mental activity). This activity occurs during the shallower stages of sleep entry during NREM sleep and during the longer, more pronounced REM sleep periods when dreams last. Because sleep can have a significant impact on memory, it is difficult to accurately and reliably describe the state of mental activity during sleep. However, we now understand that a person’s consciousness changes accordingly with the brain in different sleep stages during sleep. This gives more support to the state of consciousness hypothesis. The hypothesis suggests that people’s states of consciousness change in a template-like architecture just as brain function changes during the sleep-wake cycle. One way to study consciousness is to record both changes in the brain and changes in thoughts, and then to carefully analyze the connections between them. So, how does brain activity regulate consciousness? Directly studying the subjective experience during sleep is a very important approach in the field of consciousness research, which is currently in full swing. However, there are many problems with this approach to the study of subjective experience, which has frustrated even the most courageous scientists. And it is also important to note that the subjective opinions of individuals cannot be used as research material at all unless a great deal of effort is expended in sifting and filtering this subjective data. The average volunteer is asked to report their mental activity in the very awake state, the awake state, the just asleep state, the NREM sleep period, and the REM sleep period. Researchers rate volunteers based on their descriptions of hallucinations, thoughts, and interrelationships with the brain at that time. Hallucinations were least common in the very awake state and most common in the REM sleep period. In contrast, thinking activity was most frequent in the waking state and least frequent during REM sleep. These phenomena suggest that the brain after falling asleep is capable of both perception and thinking. However, it cannot perform both at the same time. In this sense, dreams are the same as the hallucinations and delusions known as mental illnesses. In another study researchers tested this hypothesis. They administered the Thematic Agnosia Test (TAT) to patients with schizophrenia, who scored high on ratings of cognitive discontinuity and inconsistency. Normal controls of the same age and gender as these patients had the same bizarre dreams, but the controls had far fewer such bizarre descriptions in the waking state. These findings support the hypothesis that the REM sleep period is a physiological state of the brain that creates distinctive psychotic-like mental activity. However, this function of the brain is inhibited in the waking state. In other words, the brain in the waking state is devoid of normal dreaming activity. Conversely, normal dreaming can be thought of as a normal response to highly abnormal brain and psychological conditions. It is now clear to us that consciousness is a function of the brain. Sleep and brain research is a very rare point of convergence between the fields of biological and psychological science research. The research now underway in this direction will hopefully connect molecular cell biology and behavioral consciousness science, and could even help people treat physical and mental illnesses.