Tuning for Criticality : A New Hypothesis for Sleep

We propose that the critical function of sleep [3] is to prevent uncontrolle d n uronal feedback while allowing rapid responses and prolonged retention of short-term memories. T he goal of learning is optimal behavior, and this sometimes requires the integration of sensory stimuli that ar e widely separated in time. At a neuronal level, this corresponds to persistent activity in local netwo rks. Unfortunately, when a network exhibits persistent activity, small changes in the parameters or conditions ca n lead to runaway oscillations. Thus, the very changes that improve the processing performance of the network can put it at risk of runaway oscillation. To prevent this, stimulus-dependent plasticity should only b e permitted when there is a margin of safety around the current network parameters. We propose th at a critical role of sleep is to establish a margin of safety by exposing the network to a variety of conditions and in puts, observing for erratic behavior, and adjusting the parameters accordingly. During wakefulnes s this margin of safety is gradually consumed, ultimately requiring refreshment by another period of sleep. Wh en sleep is not possible, an emergency mechanism comes into play to prevent runaway oscillations; this is done at the expense of processing efficiency, and constitutes tiredness. We contend that this theory matches th e phenomenology of sleep and tiredness better than do alternative theories, such as the memory consolidatio n theory [1, 2, 5]. This theory also makes a number of novel testable predictions.