Why do fruit fly animals need to sleep? After all, sleep disconnects them from their environment, puts them in danger, and prevents them from looking for food or companions for most of the day.

Two sleep researchers from the University of Wisconsin School of Medicine and Public Health say their hypothesis of synaptic sleep homeostasis, or “Shyness,” challenges the theory that sleep strengthens brain connections.

The SHY hypothesis, which has decades of experience in human and animal research, says sleep is important because it reduces connections between brain cells to conserve energy, prevent cellular stress, and maintain the ability of neurons to respond to selective criteria for stimuli.

“Sleep is the price the brain has to pay for learning and memory,” says Dr. Giulio Tononi of UW’s Sleep and Knowledge Center. “During wakefulness, research strengthens synaptic connections in all parts of the brain, increasing the need for energy and saturating the brain with new information. Sleep allows the brain to reboot, helping to integrate newly learned material into consolidated memories so the brain can start again the next day.

Tononi and her co-author Dr. Chiara Cirelli, both professors of psychiatry, explain their hypothesis in an article in today’s newspaper Neuron. His laboratory studies of sleep and consciousness in animals range from fruit flies to humans; SHY takes into account data from molecular, electrophysiological and behavioral studies, as well as computer simulations. The term “synaptic homeostasis” refers to the brain’s ability to balance the strength of the connections within its neurons.

Why does the brain have to reboot? Suppose someone was using their waking hours while learning a new skill such as cycling. The circuits involved in learning will be heavily reinforced, but the next day the brain will have to pay attention to learning a new problem. As such, these “cycling patterns” should be shortened so that they do not interfere with learning the new day.

“Sleep helps the brain to normalize synaptic resistance again based on a complete sample of its general knowledge of the environment,” says Tononi, “instead of being influenced by certain factors on a certain day.

In addition, the reason we also remember how to ride a bike after a night’s sleep is that the number of active circuits decreases less than those that were not actively involved in the study. Of course, there is evidence that sleep improves important characteristics of memory, including acquisition, consolidation, core idea extraction, integration, and “intellectual forgetfulness,” which allows the brain to rid itself of the inevitable accumulation of small details.

However, it is widely believed that sleep helps memory to strengthen the nervous systems during training while awake. But Tononi and Cirelli believe that memory consolidation and integration, as well as the restoration of learning ability, stem from sleep’s ability to reduce synaptic power and improve signal-to-noise ratios.

While the article provides testable evidence for the SHY hypothesis, it also notes that a number of questions remain open. One question is whether the brain can achieve synaptic homeostasis while awake if only a few circuits are busy and the rest are off to reboot itself.

Other areas for future research include sleep-specific REM function (when you tend to dream the most) and possibly the crucial role of sleep during development, intense study time, and large-scale brain remodeling.