By Leslie Alan Horvitz
We spend about a third of our lives sleeping. But do we know why we do it? Is it for biological, psychological, or evolutionary reasons? Much research has based its findings on what happens when we don’t sleep enough. The effects of sleeplessness aren’t difficult to pinpoint. Still, apart from a scientific consensus that sleep is essential to several brain functions and plays some kind of “housekeeping role,” scientists have yet to determine why.
As contended by the National Institute of Neurological Disorders and Stroke, an arm of the National Institutes of Health and the nation’s leading funder of brain research, “its biological purpose remains a mystery.”
Several theories have been advanced to explain sleep. Although these theories may be valid, none offer a complete explanation.
Why We Sleep: Many Theories
According to a theory proposed by the National Library of Medicine, sleep is essential for energy conservation. This is because our metabolic rate drops during sleep, resulting in daily energy savings of up to 15 percent. This theory argues that sleep was an evolutionary development, reducing energy use during those times—mainly at night—when it was less practical to hunt for food. On the other hand, sleep left our ancestors more vulnerable to predators.
Likewise, the restorative theory argues that sleep is necessary for the body to undertake improvements and regrowth, including muscle repair, protein synthesis, tissue growth, and hormone release. Some sleep researchers have hypothesized that without enough sleep, the brain may not be able to clear toxins effectively.
Still, a 2024 study published in Nature Neuroscience suggests this theory could be wrong. The study found that the clearance and movement of fluid in the brains of mice were markedly reduced during sleep. However, this was considered only indirect evidence.
A third theory focuses on brain plasticity—the ability of the brain to change and repair itself. Sleep allows the brain’s glymphatic system—or waste clearance system—to remove toxins from the brain that build up during the day. This lets the neurons, or nerve cells, reorganize to ensure proper brain function during waking hours.
According to Harvard Medical School’s Division of Sleep Medicine, “…brain plasticity is not entirely understood, but its connection to sleep has several critical implications. It is becoming clear, for example, that sleep plays a critical role in brain development in infants and young children. Infants spend about 13 to 14 hours per day sleeping, and about half of that time is spent in REM sleep, the stage in which most dreams occur.”
The synaptic homeostasis theory focuses on the brain’s consolidating and culling functions during sleep. While we sleep, the memories we need to preserve are converted from short-term to long-term, a process involving heightened activity in the hippocampus. Those that are irrelevant or unnecessary are discarded—a nightly decluttering operation.
The number of synapses increases during the day due to activity. Synapses are the gaps where neurons meet and use chemicals to send signals through the nervous system. If allowed to accumulate, the brain would become overloaded, like a computer’s hard drive running out of memory, making it necessary to prune the unnecessary synapses. Numerous studies supporting this theory show a pattern of synapses in experimental animals shrinking during sleep and expanding during wakefulness.
A 2024 study published in Science by György Buzsáki, professor of neuroscience at New York University, focuses on sudden and powerful high-frequency brain waves known as “sharp wave ripples,” produced by the firing of many thousands of neurons within milliseconds of one another. They are “like a fireworks show in the brain,” said Wannan Yang, a doctoral student in Buzsáki’s lab. The sharp wave ripples fire when the mammalian brain rests, whether during a break between tasks or sleep. They are known to be involved in consolidating and storing memories.
The new research shows that they’re also involved in selecting and “tagging” the high-priority memories, which are replayed in sleep while ignoring those without priority. Scientists believe resting and sleeping are vital to consolidating and retaining information. If you sleep all the time, you won’t be able to form new memories. But if you stay awake all the time, you won’t form them, either. “If you just run one algorithm, you will never learn anything,” Buzsáki said. “You have to have interruptions.”
A 2024 report in Current Biology underscores the relationship between learning and a good night’s sleep, but from a different perspective: the changes in how brain cells are connected. Some synapses are in brain regions associated with learning and memory, and sleep deprivation hurts these synapses.
Theories explaining these connections have primarily been based on the belief that synapses are identical. However, Seth Grant, a neuroscientist at the University of Edinburgh, and his team and others have found that synapses are surprisingly diverse in structure, the composition of proteins in the neurons surrounding them, and the types of neurotransmitters they use to send signals. Grant and his team call the set of synapses the “synaptome.” In mice studies, the researchers allowed one group to get about six hours of sleep while prodding the second group to keep them awake. Although the total number of synapses remained constant in both groups, the diversity of subtypes fell in the sleep-deprived mice, especially in areas of the brain associated with learning and memory: the cortex and hippocampus.
Quality sleep can also improve emotional well-being. Activity during sleep increases in certain parts of the brain—especially the amygdala, which is involved in emotional regulation and the response to fear. Research suggests that sleep deprivation causes the amygdala to overreact, heightening stress and anxiety, even when there is no fearful stimulus.
Similarly, sleep heightens activity in the striatum, which has several decision-making functions such as motor control, emotion, habit formation, and reward. Sleep also heightens activity in the
(Leslie Alan Horvitz is an author and journalist specializing in science. His nonfiction books include Eureka: Scientific Breakthroughs That Changed the World, Understanding Depression with Dr. Raymond DePaulo of Johns Hopkins University, and The Essential Book of Weather Lore. His articles have been published by Travel and Leisure, Scholastic, Washington Times, and Insight on the News, among others. He has served on the board of Art Omi, is a member of PEN America, and contributes to the Observatory. Horvitz is based in New York City. He is a contributor to the Observatory. You can find him online at lesliehorvitz.com. This article was produced by Earth | Food | Life, a project of the Independent Media Institute.
