Abrupt and Gradual Changes in Neuronal Processing upon Falling Asleep and Awakening

The neural processes associated with the transition into sleep remain poorly understood, and it is unclear whether they develop abruptly or gradually. By examining spontaneous and sound-evoked neuronal activity in the auditory cortex of rats during thousands of sleep-wake transitions, we found that stimulus-induced neuronal silence (OFF periods) emerged abruptly within seconds of sleep onset, while increases in local population synchrony unfolded gradually over tens of seconds. EEG activity mirrored this dichotomy, with abrupt changes in auditory-evoked potentials and a slow build-up of slow-wave activity. Our findings suggest that transitions between wakefulness and sleep are associated with distinct neuronal mechanisms, involving both rapid, discrete shifts and more gradual changes in neural processing