why sleep?

The Scientist | Why do we sleep? For some researchers who study memory, the findings support a popular theory that the purpose of sleep is to replay and consolidate memories from the previous day. To them, sleep is important for memory, and the deep, slow waves seen in the same part of the brain used in a task indicate that the brain circuits involved in the task are reactivating. Such reactivation, or "replay," could explain why participants perform the task with greater accuracy after a night of sleep.

But for Cirelli and Tononi, their findings suggested an entirely different—and controversial—theory was perhaps true. Sleep's core function, Cirelli and Tononi say, is to prune the strength or number of synapses formed during waking hours, keeping just the strongest neuronal connections intact. Synapse strength increases throughout the day, with stronger synapses creating better contact between neurons. Stronger synapses also take up more space and consume more energy, and if left unchecked, this process—which Cirelli and Tononi believe occurs in many brain regions—would become unsustainable.2,3 Downscaling at night would reduce the energy and space requirement of the brain, eliminate the weakest synapses, and help keep the strongest neuronal connections intact. This assumption is based on the principle in neuroscience that if one neuron doesn't fire to another very often, the connection between the two neurons weakens. By eliminating some of the unimportant connections, the body, in theory, eliminates background connections and effectively sharpens the important connections.

It's unclear how slow waves could affect synaptic strength at a molecular level, but Cirelli and Tononi suspect the slow-wave activity triggers a weakening of synapses, and the more slow waves, the more subsequent downscaling. Their belief stems from the timing of the slow waves, which swell early in the night and taper off. Plus, molecular and electrophysiological evidence indicates synapses are stronger at the beginning of the night and weakest after a long bout of sleep. To Cirelli and Tononi, the weakening of synapses overnight—which could also theoretically help people perform better on a task the next day—is the ultimate purpose of sleep.