Differential Nap-To-Nap Stability of Sleep Spindles, Slow Waves, and their Temporal Coupling: An Exploratory Study

Slow waves and sleep spindles characterise non-rapid eye movement (NREM) sleep and support cognitive and plasticity-related functions. While their stability across nights is well established, less is known about their consistency across daytime naps. Nineteen healthy young adults (20-27 years) underwent two 90-min afternoon naps, interleaved by 1 week, under polysomnographic recording. Slow waves and sleep spindles were detected using standardised algorithms, and their key features, as well as NREM spectral power, were extracted from N2 sleep and assessed for nap-to-nap reliability using intraclass correlation coefficients. Temporal coupling between spindles and slow wave events was also evaluated. Spectral power showed good consistency in the sigma band and moderate consistency in the delta band. Spindle frequency, density, and duration were highly reliable, particularly for fast spindles. Slow wave density and slope showed moderate stability, while amplitude and duration were less stable and not consistent, respectively. The phase of slow wave-spindle coupling did not show consistency between naps. Overall, some specific features of NREM sleep, particularly those related to spindles, appear relatively stable across naps and may reflect trait-like aspects of individual sleep physiology. In contrast, coupling dynamics appear more variable and influenced by state-related factors.