Sleep is pervasive, universal, irresistible, tightly regulated, and its loss impairs performance and cognition. Sleep is thought to perform essential restorative functions for the brain, and increasing evidence suggests that a key function may be to rebalance cellular changes triggered by plasticity during wake. This evidence is consistent with the hypothesis that sleep and wake may occur, be regulated, and perform their functions at the level of individual neurons. Recently, using multi-array recordings in freely moving rats, we have obtained direct evidence that sleep can occur locally within a group of cortical neurons, while the rest of the brain remains awake, and that such "local sleep" increases with the duration of wake. In this proposal, we will use multi-array recordings to establish whether OFF periods during wake ("local sleep") occur in mice, whether it increases with wake duration and whether it is associated with slow/theta waves in the local EEG and impaired performance, providing the rationale for the use of high density EEG in humans in Project III. We will then test whether local sleep, like sleep proper, is regulated by intense synaptic plasticity ("tiredness") or instead by mere activity ("fatigue"). To do so we will first establish if local sleep increases with locally induced intense plasticity. We will induce local "tiredness" in one sensorimotor cortex using both a learning task (reaching) and synaptic potentiation through electrical stimulation, and compare the effects on local sleep on the ipsi- vs. contralateral side. Next, we will determine the effects on local sleep of local opto-pharmacogenetic activation (intense activity with little plasticity), expected to lead to "fatigue," during both wake and sleep, again comparing the results to the contralateral side. Finally, we will use SBF-SEM to test whether there are ultrastructural signatures that distinguish between neurons that have been kept awake and those that have slept, and between intense plasticity ("tiredness") and intense activity ("fatigue"). Altogether, these studies in mice will complement those in flies in Project I, which use similar methods. Together, they will establish if sleep and wake are regulated homeostatically at the single neuron level, and if they leave ultrastructural signatures that reflect their consequences and functions for individual cells.