A better question than “Why do we sleep?” may be “Why didn’t we develop night vision?” considering how we are essentially paralyzed mid-slumber. But scientists researching why we sleep have found the vulnerable state to be particularly helpful in a number of brain functions, most recently the SHY hypothesis, which claims that if our brain is expected to absorb all the information it’s learned, it, like us, needs a rest.

Scientists from the University of Wisconsin have developed the hypothesis, formally known as the synaptic homeostasis hypothesis, as a means to explain the function of sleep. When we interact with our daily world, our brains play receiver to countless packets of data: numbers, ideas, thoughts, feelings, sensory information. This data goes somewhere, and depending on the type of information it is, typically we want it to stay there. So in order for that to happen, the research team suggests, the brain needs to allow its connections to weaken, paradoxically, so energy isn’t continuously being devoted to data storage.

"Sleep is the price the brain must pay for learning and memory," Dr. Giulio Tononi, of the UW Center for Sleep and Consciousness, said in a statement. "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. "

What this means, basically, is that during the daytime the brain is tasked with extraordinary powers of computation. It processes people’s faces, learns new names, remembers which foods taste terrible, keeps track of our habits, and so on. To do this, it must keep a log of all these newly learned facts and experiences, which fall into one of two categories: semantic and episodic memories. Semantic memories are the facts you learned in high school and, miraculously, can still recall today. Episodic memories are the intricate stories of your past — the history teacher, perhaps, who made those semantic memories so memorable. For the brain to keep these memories stable over time, first they must be incorporated with existing memories.

The problem is, the brain is really good at keeping memories (semantic ones, especially) in the same place. The neural connections are too strong, scientists are finding, to allow new data to penetrate the dense web of memories in a waking state. When we sleep, the connections — called synapses — weaken, allowing new information to seep in. Think of your brain as a soup, one that you freeze each night to keep fresh. In the morning, if you decide to add another ingredient, first you have to reheat the soup, weakening its overall structure as a solid. Only after you turn it into a liquid can you add the extra ingredient, before finally putting it back in the freezer to incorporate overnight.

What’s more, if you don’t give your brain enough time to fully absorb and internalize the data you’ve accumulated, it won’t necessarily keep it all. Similar to the flavors of an improperly stored soup, your memories will be less bold because they haven’t set right. "Sleep helps the brain renormalize synaptic strength based on a comprehensive sampling of its overall knowledge of the environment," Tononi said, "rather than being biased by the particular inputs of a particular waking day." In other words, sleep helps you remember everything, not just what’s recent.

Contrary to other theories, which peg sleep as a function of waste removal and argue that waking states are physically unable to do this, the present researchers say they don’t know for sure whether sleep is necessary to this function — only that it has only been observed during sleep. Non-REM sleep and certain waking states may be capable of producing the same weak connections, the team points out.

“For example, it could be that a given time only a small subset of brain circuits are engaged by behavior, and all other circuits are effectively online even in wake. Conceivably, such offline circuits could renormalize synaptic strength even while the organism is behaving.”

Source: Tononi G, Cirelli C. Sleep and the Price of Plasticity:From Synaptic and Cellular Homeostasis to Memory Consolidation and Integration. Neuron. 2014.