Epidemiological perspectives on sleep : health-related outcomes and subjective-objective sleep assessments

Abstract

This thesis aimed to expand knowledge about self-reported habitual sleep in relation to health outcomes and objectively measured sleep.

Study I was a cross-sectional analysis of 40,197 Swedish adult volunteers participating in the National March, a fundraising event for the Swedish Cancer Society held in 1997. We compared the entire distribution of BMI between subjects with different sleep patterns. Relative to those who reported 6–8 h or good-quality sleep, the upper tail of the BMI distribution, representing the heaviest 10% of the population, was extended towards higher values by 0.39– 1.79 kg/m2 among subjects with ≤5 h, ≥9 h or poor-quality sleep. The medians were similar. The extension of the upper tail without a corresponding change in the central tendency suggests that unfavorable sleep patterns are associated with BMI in a subset of people.

Study II examined sleep duration and insomnia symptoms (difficulty falling asleep or maintaining sleep, early morning awakening, and nonrestorative sleep) in relation to risk of cardiovascular events (incident myocardial infarction, stroke or heart failure, or death from all cardiovascular diseases). During a follow-up of 13.2 y among the same volunteers as in study I (n = 41,192), subjects who reported sleeping ≤5 h showed an increased risk of cardiovascular events relative to those who slept 7 h (adjusted hazard ratio = 1.24; 95% confidence interval, CI: 1.06–1.44). Additional adjustment for BMI, self-rated health, and other pertinent factors attenuated this relationship. We observed no excess risk among those who slept 6 h, ≥8 h, or who had insomnia symptoms. Thus, no independent association was found between sleep habits and incident cardiovascular events.

Study III tested whether subjects with and without obstructive sleep apnea syndrome (OSAS) could be accurately distinguished from each other using self- report symptoms typical of the disease obtained from the Karolinska Sleep Questionnaire (KSQ). Among 103 subjects referred to a large sleep clinic in Stockholm, 60% had OSAS. Sensitivity and specificity of self-reported apnea/snoring symptoms were 0.56 (95% CI: 0.44–0.69) and 0.68 (0.52–0.82). Corresponding figures for self-reported sleepiness symptoms were 0.37 (0.25– 0.50) and 0.71 (0.55–0.84). Diagnostic accuracy of apnea/snoring and sleepiness symptoms reported in the KSQ was poor; clinical use cannot be recommended.

Study IV analyzed the association of sleep quality and restoration from sleep reported in the KSQ with standard polysomnography parameters recorded on multiple occasions in 31 adults without sleep problems. Stage 2 sleep predicted worse sleep quality and slow-wave sleep predicted better sleep quality. Slow- wave sleep was also related to less subjective restoration from sleep, but this association disappeared with adjustment for age. We found some evidence in support of polysomnographic correlates of self-reported habitual sleep quality.