Maintaining wakefulness is associated with a progressive increase in the need for sleep. This phenomenon has been linked to changes in synaptic function. The synaptic adhesion molecule Neuroligin-1 (NLG1) controls the activity and synaptic localization of N-methyl-Daspartate receptors, which activity is impaired by prolonged wakefulness. We here highlight that this pathway may underlie both the adverse effects of sleep loss on cognition and the subsequent changes in cortical synchrony. We found that the expression of specific Nlg1 transcript variants is changed by sleep deprivation in three mouse strains. These observations were associated with strainspecific changes in synaptic NLG1 protein content. Importantly, we showed that Nlg1 knockout mice are not able to sustain wakefulness and spend more time in nonrapid eye movement sleep than wild-type mice. These changes occurred with modifications in waking quality as exemplified by low theta/alpha activity during wakefulness and poor preference for social novelty, as well as altered delta synchrony during sleep. Finally, we identified a transcriptional pathway that could underlie the sleep/wake-dependent changes in Nlg1 expression and that involves clock transcription factors. We thus suggest that NLG1 is an element that contributes to the coupling of neuronal activity to sleep/wake regulation.ChIP | EEG | gene expression | sleep homeostasis | synaptic plasticity S leep is crucial for learning, memory, and other functions essential for proper functioning of the brain and body (1, 2). These functions have been associated with the sleep recovery process, which defines a level of pressure for sleep that increases with wakefulness and dissipates during sleep and that is reflected by changes in sleep intensity (3, 4). Sleep intensity is indexed by electroencephalographic (EEG) markers of neuronal synchrony in delta frequencies (1-4 Hz) measured during nonrapid eye movement (NREM) sleep (5). During wakefulness, mechanisms favoring desynchrony in the delta range predominate, and the brain can maintain cognition, whereas during sleep, events promoting network synchrony mostly take place with high delta activity thought to be permissive of recovery (3, 6). The sleep recovery process has been hypothesized to originate and contribute to the maintenance of both synaptic and network equilibrium (6-8). This notion is supported by the observation that specific plasticityrelated genes may be directly involved in regulating sleep need (9). Certain clock genes may also directly contribute, in a circadian-independent manner, to the sleep recovery process (5, 9). However, the mechanisms underlying the capacity and requirement of the brain to switch from an alert desynchronized state to an unconscious synchronized state remain elusive.Glutamate, the main excitatory neurotransmitter of the brain, can induce long-term modifications of synaptic transmission and, thus, changes in network connectivity. This is achieved mainly via glutamate's action on two types of receptors: N-methyl-D-aspa...