Reassessing the validity of slow-wave dynamics as a proxy for NREM sleep homeostasis

Sleep-wake driven changes in NREM sleep (NREMS) EEG delta ({delta}: [~]0.75-4.5Hz) power are widely used as proxy for a sleep homeostatic process. We noted frequency increases in {delta}-waves in sleep-deprived (SD) mice, prompting us to re-evaluate how slow-wave characteristics relate to prior sleep-wake history. We discovered two types of {delta}-waves; one responding to SD with high initial power and fast, discontinuous decay ({delta}2: [~]2.5-3.5Hz) and another unrelated to time-spent-awake with slow, linear decays ({delta}1: [~]0.75-1.75Hz). Human experiments confirmed this {delta}-band heterogeneity. Similar to SD, silencing of centromedial thalamus neurons boosted {delta}2-waves, specifically. {delta}2-dynamics paralleled that of temperature, muscle tone, heart-rate, and neuronal UP/DOWN 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. These short-lasting {delta}2-dynamics challenge accepted models of sleep regulation and function based on the merged {delta}-band as sleep-need proxy.