A hippocampal 'sharp-wave sleep' state that is dissociable from cortical sleep

Cortical slow waves reflect the need for sleep, and their presence indicates a state of disconnection and homeostatic regulation. However, little is known about the neural signatures of sleep need beyond the cortex. Here we performed chronic, continuous, 48-h Neuropixels recordings in male rats to capture hippocampal activity over sleep/wake cycles. We show that hippocampal sharp waves (SPWs) and, to some extent, ripples and dentate spikes (DSs) closely reflect sleep need. Hippocampal SPWs occurred during behavioral sleep and, unlike cortical slow waves, also during quiet wake. The expression of hippocampal SPW, ripple and DS during cortical wakefulness was negatively correlated with that during subsequent cortical sleep, suggesting that these events fulfill similar homeostatic functions. Moreover, the slow-to-fast gamma ratio was always high during SPW, consistent with a switch to a partially disconnected mode. We propose that SPWs define a partially disconnected, homeostatically regulated, unitary state of the hippocampus, which we refer to as 'hippocampal sharp wave sleep'.