Characterization of overnight slow-wave slope changes across development in an age-, amplitude-, and region-dependent manner.

Study Objectives: The restorative function of sleep has been linked to a net reduction in synaptic strength. The slope of slow-waves, a major characteristic of non-rapid eye movement (NREM) sleep, has been shown to directly reflect synaptic strength, when accounting for amplitude changes across the night. In this study, we aimed to investigate overnight slope changes in the course of development in an age-, amplitude-, and region-dependent manner.

Methods: All-night high-density electroencephalography data were analyzed in a cross-sectional population of 60 healthy participants in the age range of 8-29 years. To control for amplitude changes across the night, we matched slow-waves from the first and the last hour of NREM sleep according to their amplitude.

Results: We found a reduction of slow-wave slopes from the first to the last hour of NREM sleep across all investigated ages, amplitudes, and most brain regions. The overnight slope change was largest in children and decreased toward early adulthood. A topographical analysis revealed regional differences in slope change. Specifically, for small amplitude waves the decrease was smallest in an occipital area, whereas for large amplitude waves, the decrease was smallest in a central area.

Conclusions: The larger slope decrease in children might be indicative of a boosted renormalization of synapses during sleep in childhood, which, in turn, might be related to increased plasticity during brain maturation. Regional differences in the extent of slow-wave slope reduction may reflect a "smart" down-selection process or, alternatively, indicate amplitude-dependent differences in the generation of slow-waves.