Study objectives: Clinical rating scales, self-reports, and diagnostic instruments measuring depression often inquire about daytime fatigue and tiredness. Excessive daytime sleepiness refers specifically to the tendency to feel drowsy or fall asleep during waking hours and is considered conceptually and operationally independent from the fatigue, tiredness, and sleeping difficulties that characterize depression. The objective of this study was to examine whether daytime sleepiness assessed using the Epworth Sleepiness Scale and depressive symptoms assessed using the Geriatric Depression Scale are genetically related.
Design/setting: Cross-sectional data were collected via questionnaire in 1998-2000.
Participants: Population-based sample of more than 5,000 male elderly twins aged 69-82 years old at the time of survey.
Interventions: N/A.
Measurements and results: There was evidence for moderate heritability for daytime sleepiness (36.9%) and depressive symptoms (30.7%). There was evidence for a significant genetic correlation (0.40) between the 2 measures, suggesting that both daytime sleepiness and depressive symptoms have some genes in common. The genetic correlation was reduced to 0.21 after adjustment for several covariates.
Conclusions: The results showed that the often reported phenotypic correlation between daytime sleepiness and depressive symptoms is due, in part, to modest overlap in genetic factors, at least in elderly men. However, the majority of individual variation in daytime sleepiness and depressive symptoms, in particular after covariate adjustment, was attributable to individual-specific environmental factors.