Rapid fast-delta decay following prolonged wakefulness marks a phase of wake-inertia in NREM sleep

Sleep-wake driven changes in non-rapid-eye-movement sleep (NREM) sleep (NREMS) EEG delta (delta-)power are widely used as proxy for a sleep homeostatic process. Here, we noted frequency increases in delta -waves in sleep-deprived mice, prompting us to re-evaluate how slow-wave characteristics relate to prior sleep-wake history. We identified two classes of delta -waves; one responding to sleep deprivation with high initial power and fast, discontinuous decay during recovery sleep (delta 2) and another unrelated to time-spent-awake with slow, linear decay (delta 1). Reanalysis of previously published datasets demonstrates that delta -band heterogeneity after sleep deprivation is also present in human subjects. Similar to sleep deprivation, silencing of centromedial thalamus neurons boosted subsequent delta 2-waves, specifically. delta 2-dynamics paralleled that of temperature, muscle tone, heart rate, and neuronal ON-/OFF-state lengths, all reverting to characteristic NREMS levels within the first recovery hour. Thus, prolonged waking seems to necessitate a physiological recalibration before typical NREMS can be reinstated. Changes in EEG delta-activity are widely used as proxy of sleep propensity. Here the authors demonstrate in mice and humans the presence of two types of delta-waves, only one of which reports on prior sleep-wake history with dynamics denoting a wake-inertia process accompanying deepest non-rapid-eye-movement sleep (NREM) sleep.