Characteristics of study population by sleep QUALITY¹ tertiles.

1Total sleep quality score divided into tertiles. 1st tertile = least disturbance; 3rd tertile = most disturbance. See Methods for further details. 2Results expressed as mean (sd) or %. 3ANOVA for continuous data, chi-square for categorical data (where p value represents differences between all categories). 4See Methods section for description of variables.