Sleep deprivation results in reduced cognitive function coincident with regional 'sleep-like' slow waves and theta waves in the brain. Here, single-neuron activity and local field potentials (LFPs) were recorded in sleep-deprived and non-sleep-deprived human neurosurgical patients while they performed a task that required them to distinguish between face and non-face pictures. In the sleep-deprived group, a subpopulation of medial temporal lobe (MTL) neurons showed fewer spikes of lengthened duration and delayed onset shortly before the onset of cognitive lapses. During these cognitive lapses, there was an increase in local slow and theta activity of LFPs, indicating that local, state-dependent activity changes in MTL are associated with cognitive lapses.