Cortical firing dynamics during micro-arousals vary with sleep/wake history and micro-arousal duration

Non-rapid eye movement (NREM) sleep is not uniform but characterized by brief intrusions of wake-like brain activity and increased muscle tonus known as micro-arousals or brief awakenings. Although micro-arousals are an inherent feature of human and animal sleep, knowledge about the neural correlates of micro-arousals is sparse. We here developed an algorithm for automatic detection of micro-arousals based on EMG activity and used it to analyse the associated laminar neural activity in the motor cortex in mice. Our analysis showed that short micro-arousals with a duration below 5 s were associated with decreased cortical firing, while longer micro-arousals with a duration of 5–10 s were associated with increased firing in a similar manner to transitions to wakefulness. Analysis of single-channel firing showed that some channels exhibited increased activity immediately prior to micro-arousals, while others exhibited decreased activity. Slow wave activity (SWA, 1–4 Hz) immediately after micro-arousals was tightly correlated with sleep pressure and even surpassed average SWA levels during NREM sleep in sleep deprived animals. This study provides new insights into the neural mechanisms associated with micro-arousals and identifies a new link between sleep architecture and sleep homeostasis.