Why do humans and other animals need sleep? Despite being an activity that literally every human on the planet engages in, sleep remains one of the stranger, lesser understood phenomena of human behavior. Earlier this year researchers from the University of Wisconsin compiled a body of evidence in support of one of the established hypotheses for why humans and other animals need sleep. It is called the “synaptic homeostasis hypothesis” or “SHY hypothesis” for short, and paradoxically claims that sleep strengthens a learning brain by weakening the synapses between individual neurons.
Each day, no matter how routine, requires a brain to learn, store memories, think critically, and make decisions. An insurmountable amount of data reaches the human brain with each second. Such data ranges from information that individuals are consciously aware of (such as reading written words on a page) to information that remains in the background of self-awareness (such as how the body is positioned in a seat). This learning process increases both the strength and number of connections (synapses) between the individual neurons that make up much of the brain and the rest of the nervous system.
Given that sleep helps improve memory and overall cognitive performance, one might reasonably expect that sleep might somehow act as a kind of glue that helps solidify all of the connections made between neurons during the waking day. However researchers from the Department of Psychiatry at the University of Wisconsin Madison have taken the exact opposite stance. They propose that sleep actually helps strengthen the mind by weakening the synaptic connections between neurons.
Major support for this hypothesis comes from previously observed structural evidence. First, studies with fruit flies have shown that flies have fewer and smaller synapses after waking from several hours of sleep than they did when they first dozed off. This is especially evident in the dendrites—small information gathering projections in neurons—that pertain to neurons involved in the visual system. Such experimental data has also been replicated in mice, where it was noted that young mice in particular eliminate large numbers of their dendritic spines at night.
In application to daily life, the overall weakening of synapses during sleep allows the brain to absorb more information during the day. As Tononi and Cirelli wrote in the paper that they published in the esteemed journal Cell, “sleep is the price we have to pay for plasticity”.
If true, the synaptic homeostasis hypothesis would predict that centers of the brain used specifically in learning—not just routine use—are the key places in the brain where sleep has the greatest effect. The University of Wisconsin’s Center for Sleep and Consciousness website predicts that sleep could be locally induced (as opposed to the global sleeping experience with which the modern human is familiar). Exploration into inducing such a localized sleep state is already underway in animal models and humans. If successful, such a technique could hypothetically be a great performance enhancer for boosting cognitive performance in specifically targeted areas of the brain.
By Sarah Takushi