Why do animals ranging from fruit flies to humans all need to sleep? After all, sleep disconnects them from their environment, puts them at risk and keeps them from seeking food or mates for large parts of the day.
Two leading sleep scientists from the University of Wisconsin School of Medicine and Public Health say that their synaptic homeostasis hypothesis of sleep or "SHY" challenges the theory that sleep strengthens brain connections. The SHY hypothesis, which takes into account years of evidence from human and animal studies, says that sleep is important because it weakens the connections among brain cells to save energy, avoid cellular stress, and maintain the ability of neurons to respond selectively to stimuli.
"Sleep is the price the brain must pay for learning and memory," says Dr. Giulio Tononi, of the UW Center for Sleep and Consciousness. "During wake, learning strengthens the synaptic connections throughout the brain, increasing the need for energy and saturating the brain with new information. Sleep allows the brain to reset, helping integrate, newly learned material with consolidated memories, so the brain can begin anew the next day. "
Tononi and his co-author Dr. Chiara Cirelli, both professors of psychiatry, explain their hypothesis in a review article in today's issue of the journal Neuron. Their laboratory studies sleep and consciousness in animals ranging from fruit flies to humans; SHY takes into account evidence from molecular, electrophysiological and behavioral studies, as well as from computer simulations. "Synaptic homeostasis" refers to the brain's ability to maintain a balance in the strength of connections within its nerve cells.
Why would the brain need to reset? Suppose someone spent the waking hours learning a new skill, such as riding a bike. The circuits involved in learning would be greatly strengthened, but the next day the brain will need to pay attention to learning a new task. Thus, those bike-riding circuits would need to be damped down so they don't interfere with the new day's learning.
"Sleep helps the brain renormalize synaptic strength based on a comprehensive sampling of its overall knowledge of the environment," Tononi says, "rather than being biased by the particular inputs of a particular waking day."