Why do humans sleep? Scientists have yet to find a definite answer to this question.

Source: Science and Technology Daily 
A recent paper published by the research team of Stanford University School of Medicine in the journal Nature pointed out that the neural activity patterns of zebrafish during sleep are similar to those of humans. This implies that sleep activities may have evolved at least 450 million years ago, when animals on Earth were still all living in the sea.
Eating and sleeping - these are things that everyone does every day. But who has ever thought about why humans need to sleep? This seems like an extremely simple question, but the answer is much more complex than you might imagine. Over the years, scientists have made many speculations and conjectures to explain why humans need to sleep every night, but there is still no unified and definite answer.
Is sleep for energy conservation?
Among the speculations about the reasons for sleep, the energy conservation theory is relatively well-known. This view holds that sleep is merely a way to conserve energy. Professor Lei Xu, a young top talent in the "Thousand Talents Program" and the director of the Sleep Neuroimaging Center at Southwest University, told Science and Technology Journalist, "In 1975, American neuroscientist Berg proposed that the purpose of sleep is to lower the basal metabolic rate, allowing the energy obtained to be preserved and the energy lost during the day to be restored. Animals also enter hibernation from slow-wave sleep, and the characteristics of their slow-wave sleep are highly similar to those of human sleep, suggesting that sleep may have the same purpose as hibernation - energy conservation. "However, the opposition to this view is very strong at present. An opposite view holds that sleep can save approximately 5% - 10% of energy, at most no more than 15%, and the saved energy is very limited. The restoration and maintenance theory may be too rough and simple." Lei Xu said.
Indeed, sleep does help us save energy, which is crucial for animals struggling to survive on the brink of starvation and for primitive humans. During food-scarce seasons, animals will extend their sleep time or even start hibernation to conserve energy. However, as evolution progresses, some species no longer constantly struggle to fill their stomachs, such as humans. Some even say that sleeping for one night can only help humans save about 110 calories, which is equivalent to the calories of a hot dog. So, does that mean that if people eat one more hot dog every day, they can stop sleeping? Clearly, the answer is no. This indicates that apart from helping to conserve energy, sleep has other more important meanings for humans. 
Is sleep for better learning and memory?
Unlike the energy conservation theory, there is also a function theory of sleep. This school of thought holds that sleep is for achieving certain functions, such as "memory consolidation". Many sleep deprivation experiments have shown that sleep affects the consolidation of long-term memory. Whether it's animals or humans, if they sleep after learning new knowledge and reviewing previously learned knowledge, the memory effect will be much better.
"The function theory of sleep has gained wide recognition. Currently, the academic community generally believes that sleep is beneficial for the repair and growth of neural tissues to ensure the normal functioning of the body during the next day," said Lei Xu. Sleep has a protective and consolidating effect on memory, and good sleep is also important for the relearning the next day. Through technical means such as neuroimaging, people have discovered more details of memory being protected during sleep. During sleep, the encoded memory information during the day is reactivated and replayed. These memory reactivations are closely related to the specific brain waves during sleep, including alpha waves, theta waves, and delta waves.
People have also gradually realized that different sleep times result in different types of consolidated memories. For example, in the first half of the night, there are more slow-wave sleep, mainly consolidating declarative memory and spatial memory. "We suggest that teenagers should go to bed early and avoid staying up late. In fact, this is very helpful for strengthening declarative memory such as knowledge and formulas," said Lei Xu. In the second half of the night, there is more rapid eye movement sleep, which is related to the consolidation of procedural memory and emotional memory. "This is why some patients with emotional disorders often have more terrifying dreams in the second half of the night, even having nightmares," he said.
In recent years, the "synaptic homeostasis theory" has emerged as a new force. "The synaptic homeostasis theory is actually an explanation of the sleep learning and memory function theory. It mainly explains what happens in the brain during the consolidation of memory," said Lei Xu. The synaptic homeostasis theory holds that synapses, as the channels for information transmission between neurons, receive new information constantly when the brain is awake. To adapt to the demand for information transmission, synaptic activities continuously increase. However, synapses cannot be constantly active; otherwise, there will be no space to form new memories. During sleep, due to the relative isolation from external stimuli, synapses receive sufficient rest during this period. The relative contraction of synapses prepares the body for the input of information for the next day and the learning of new things. In fact, to explain the relationship between sleep and memory consolidation, some scholars have proposed the system consolidation hypothesis. This hypothesis holds that memory consolidation mainly relies on the cooperation of the prefrontal lobe and the hippocampus to complete. The hippocampus is responsible for temporarily storing new memories formed during the day, while during sleep, the prefrontal lobe distributes the temporarily stored memories to the entire brain. This is a completely different hypothesis from synaptic homeostasis.
What other hypotheses are there about sleep?
In addition to the above-mentioned hypotheses, there are also theories about why humans sleep, such as the restoration and repair function theory and the body immunity theory.
Lei Xu explained that these theories respectively explain the functions of sleep from metabolic and immune perspectives. The restoration and repair function theory holds that sleep allows the brain and body that have worked for a day to rest, repair, and recover; while the body immunity theory discovers that sleep can increase the levels of T lymphocytes and B lymphocytes in the blood, believing that healthy sleep can help activate the immune system and improve immune and regeneration functions, etc.
Lei Xu said that regarding the reasons for sleep, each theory has some reasonable aspects, but the current understanding is that they may all be biased. "These theories may all be correct, but further research is needed to confirm them. The process of scientific exploration is actually a process of constantly negating existing theories and approaching truth." Lei Xu emphasized that current research on sleep has achieved some significant results. For instance, recent studies have shown that different sleep stages may have different functions. Deep sleep is mainly used for consolidating the brain's systems, while rapid eye movement sleep is used for synaptic consolidation. The synthesis and metabolism of many endogenous hormones are related to the sleep cycle. The growth hormone level of children increases after falling asleep, reaching its peak during the deep sleep stage. The mitosis of lymphocytes and the growth of bones are the fastest during sleep. Those who have a high level of physical activity during the day have a higher proportion of deep sleep.