2.5. Statistical Methods

The normality of distributions was assessed by the Shapiro-Wilk test.

Data are expressed as mean ± standard deviation, and t-tests were used if the variables conformed to a normal distribution (alpha, beta, theta, sigma, delta, DFA, age, TST, WASO, R/TST, N1/TST, and N2/TST). Nonnormally distributed continuous variables (fast ratio, AHI, BMI, SE (%), sleep latency, and N3/TST (%)) were analyzed using a nonparametric test and presented as the median (25th-75th percentile).

A Spearman correlation was used to analyze the correlation between cognitive impairment and power spectral analysis and DFA. The values of DFA in NREM sleep were used to diagnose cognitive dysfunction using receiver operating characteristic (ROC) curve analysis. A p value of < 0.05 indicated a significant difference.

SPSS 17.0 software (SPSS Inc., Chicago, IL, USA) was used for statistical analysis.