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

Sleep-wake driven changes in NREM sleep (NREMS) EEG delta (δ: ∼0.75-4.5Hz) power are widely used as proxy for a sleep homeostatic process. We noted frequency increases in δ-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 δ-waves; one responding to SD with high initial power and fast, discontinuous decay (δ2: ∼2.5-3.5Hz) and another unrelated to time-spent-awake with slow, linear decays (δ1: ∼0.75-1.75Hz). Human experiments confirmed this δ-band heterogeneity. Similar to SD, silencing of centromedial thalamus neurons boosted δ2-waves, specifically. δ2-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 δ2-dynamics challenge accepted models of sleep regulation and function based on the merged δ-band as sleep-need proxy.