Design and statistical analysis
This study was an experimental, randomized cross-over study, a within-subjects design with repeated measures. Each participant experienced both interventions: sleep fragmentation and sleep restriction (see Fig. Time series analysis of each laboratory night with subsequent 1-min epochs (from light-off-hour until light-on-hour) were performed. One minute was chosen as it is the shortest period to analyze LF-spectral band: 0.04 Hz corresponds to a period time of 25 s.All HRV data analysis was performed according to the Task Force on HRV measurementsTwo participants with more than 70% missing data during a night (due to missing or deficient electrocardiogram or electroencephalogram) were completely removed from the analysis, leaving 18 complete cases for analysis.Data was checked for normal distribution with histograms and Q–Q-Diagrams. Kolmogorov–Smirnov and Shapiro–Wilk-Test were not used due to the small number of cases and to avoid multiple testing. As normal distribution could not be confirmed for all parameters and sleep stages, and because of the small case number, we applied non-parametric tests.The four baseline and intervention nights (BR, R, BF, F) were compared using non-parametric Friedman-Test with adjusted Data were statistically analyzed by using the software IBM SPSS Statistics, Version 25 (IBM, Corp., Armonk, NY). The same software was also used for charts. For all statistical tests, alpha level was set at p ≤ 0.05.