The infraslow fluctuation of sigma power during sleep and its links to markers of arousal and memory reactivation across development.

Sleep is both a state of disconnection from the environment and a critical period for restoration. But how does sleep balance responsiveness with the protection of key functions? The infraslow fluctuation of sigma power (ISFS)-the clustering of sleep spindles over 10-100 s-is thought to regulate this trade-off in rodents by alternating between fragile periods of high arousability and protected periods of spindle-rich stability. However, the organization of arousal and memory reactivation markers within the human ISFS and its conservation in younger ages remain unclear. This study characterizes the ISFS from childhood to young adulthood (N = 154; ages 8-26), examining its relationship with electrophysiological markers of arousal (i.e., microarousals) and memory reactivation (i.e., slow wave-sleep spindle coupling). Results indicate that the ISFS is present across all ages, with frequency, variability, and strength increasing from early to late adolescence. Notably, electrophysiological markers of arousal and memory reactivation are organized within the spindle-rich ISFS peak. The consistent presence and organization of the ISFS suggest it may be intrinsic to sleep, with adolescence marking a dynamic window. These insights may guide interventions to promote healthier sleep across development.