Sleep microstructure correlates of the first-night effect: sleep spindles and slow-wave activity in healthy adults

The first-night effect (FNE) is a well-documented phenomenon in polysomnographic (PSG) research, yet its underlying neurophysiological mechanisms remain only partially understood. This study aims to examine changes in sleep microstructure associated with FNE, with particular focus on sleep spindles and slow-wave activity (SWA).

92 healthy participants underwent polysomnographic recordings on two consecutive nights. Sleep efficiency change (ΔSE) was used to measure FNE. Characteristics of sleep spindles (amplitude, duration, frequency, density) and relative EEG power spectral density (PSD) during slow-wave sleep (SWS) in the slow oscillation and delta bands were analyzed. Paired Wilcoxon tests compared night-to-night differences, and hierarchical Generalized Additive Models were used to evaluate nonlinear relationships ΔSE.

Night 1 showed significantly lower total sleep time (TST), lower sleep efficiency (SE), higher wake after sleep onset (WASO), and longer REM latency (RL) than Night 2. Microstructurally, although sleep spindle characteristics remained stable, the GAM analysis revealed significant nonlinear relationships on the first night: an exacerbated FNE was specifically linked to shorter spindle durations (with an inflection point around 0.9 s) and with frequencies that deviated from approximately 13.3 Hz. Relative slow oscillation (SO) power during SWS was notably higher on Night 1, while delta power was reduced.

Specific thresholds of spindle duration and frequency are essential for modulating adaptation to new sleep environments. Spindle parameters may serve as neurophysiological indicators of an individual's ability to adapt to environmental sleep changes and create targeted interventions to support sleep adaptation.