Asynchronous cortical recovery from sleep inertia: region- and frequency-specific dynamics following deep NREM sleep

Sleep inertia (SI) is a transient state of impaired alertness and cognition after awakening, typically exacerbated by prior deep sleep. Despite its prevalence, the fine-grained temporal dynamics of brain activity during this transition remains poorly understood. We employed trial-by-trial behavioral assessment (20-second resolution) to map the recovery trajectories of 44 participants following awakenings from either shallow (N1/N2) or deep (N3) NREM sleep. We found that awakening from deep NREM sleep resulted in a significantly slower return to baseline working-memory performance. This behavioral deficit was mirrored by specific EEG signatures: frontal low-frequency activity (delta/theta) returned to baseline levels within minutes, whereas posterior high-frequency activity (alpha/beta) remained altered for a prolonged period. These findings reveal that the human brain reboots asynchronously following deep NREM sleep, with a "frontal-first, posterior-last" gradient that dictates the time course of behavioral restoration.