2.4. Co-variates

Potential confounders included age, sex, body mass index (BMI), comorbidities, sleep disturbance and education. Information about age and sex was collected from medical records. Height (without shoes) and weight (in light clothing) was measured by medical students and BMI was calculated (kg/m²). Burden of comorbidities were measured by a self-administered comorbidity index (range: 0–45) [23]. Greater scores indicate higher total burden of comorbidities. The questionnaire assesses the presence of 12 predefined conditions and three additional self-reported comorbidities in addition to whether the person receives treatment for the condition and if it limits their activity level. The participants self-reported their degree of sleep disturbance by choosing one out of five statements describing either normal sleep (no problems), slight problems (e.g. difficulty in falling asleep, or sometimes waking at night), moderate problems (e.g. disturbed sleep, or feeling I have not slept enough), greater problems (e.g. having to use sleeping pills often or routinely, or usually waking at night and/or too early in the morning) or severe sleeplessness (e.g. sleep is almost impossible even with full use of sleeping pills, or staying awake most of the night) [24]. Level of education was reported on a seven-point scale, and dichotomized into lower or higher education (at least four years of university or higher education).