Simplifying a bit, a neuron's dilemma is “to fire or not to fire” or “to burst or not to burst.” Together, these energetic and informational constraints force neurons to fire sparsely and selectively: bursting only in response to a small subset of inputs while remaining silent or only firing sporadic spikes in response to a majority of other inputs ( Balduzzi and Tononi, 2013). Informationally, a neuron is a tight bottleneck: it can receive a very large number of different input patterns over thousands of synapses, but through its single axon it produces only a few different outputs. Neurons should fire sparsely and selectivelyĮnergetically, a neuron is faced with a major constraint: firing is more expensive than not firing and firing strongly (bursting) is especially expensive ( Attwell and Gibb, 2005). By renormalizing synaptic strength, sleep reduces the burden of plasticity on neurons and other cells while restoring neuronal selectivity and the ability to learn, and in doing so enhances signal-to-noise ratios (S/N), leading to the consolidation and integration of memories. Increased synaptic strength also reduces the selectivity of neuronal responses and saturates the ability to learn. In other words, sleep is “the price we pay for plasticity.” Increased synaptic strength has various costs at the cellular and systems level including higher energy consumption, greater demand for the delivery of cellular supplies to synapses leading to cellular stress, and associated changes in support cells such as glia. SHY proposes that the fundamental function of sleep is the restoration of synaptic homeostasis, which is challenged by synaptic strengthening triggered by learning during wake and by synaptogenesis during development ( Fig. Here we review a proposal concerning what this function might be - the synaptic homeostasis hypothesis or SHY ( Tononi and Cirelli, 2003, 2006). The risks inherent in forgoing vigilance, and the opportunity costs of not engaging in more productive behaviors, suggest that allowing the brain to go periodically ‘off-line’ must serve some important function. Its hallmark is a reversible disconnection from the environment, usually accompanied by immobility. Sleep occupies a large fraction of the day, it occurs from early development to old age, and it is present in all species carefully studied so far, from fruit flies to humans. However, what exactly is being restored by sleep has proven harder to explain. After a good night of sleep, brain and body feel refreshed and we are restored to normal function. Why we need to sleep seems clear: without sleep, we become tired, irritable, and our brain functions less well. This review considers the rationale and evidence for SHY and points to open issues related to sleep and plasticity. This happens through the off-line, comprehensive sampling of statistical regularities incorporated in neuronal circuits over a lifetime. Activity-dependent down-selection of synapses can also explain the benefits of sleep on memory acquisition, consolidation, and integration. During sleep, spontaneous activity renormalizes net synaptic strength and restores cellular homeostasis. This increases cellular needs for energy and supplies, decreases signal-to-noise ratios, and saturates learning. During a waking episode, learning statistical regularities about the current environment requires strengthening connections throughout the brain. But why does the brain need to disconnect from the environment for hours every day? The synaptic homeostasis hypothesis (SHY) proposes that sleep is the price the brain pays for plasticity. Sleep is universal, tightly regulated, and its loss impairs cognition.
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