Sleep quality predicts positive and negative affect but not vice versa. An electronic diary study in depressed and healthy individuals.

BACKGROUND: The exact nature of the complex relationship between sleep and affect has remained unclear. This study investigated the temporal order of change in sleep and affect in participants with and without depression.

METHODS: 27 depressed patients and 27 pair-matched healthy controls assessed their sleep in the morning and their affect 3 times a day for 30 consecutive days in their natural environment. Daily sleep quality and average positive affect (PA) and negative affect (NA) were used to examine whether changes in sleep quality preceded or followed changes in PA and NA, and whether this was different for patients and healthy controls. Second, presumptive mediating factors were investigated. We hypothesized that fatigue mediated the effect of changes in sleep quality on subsequent PA/NA, and that rumination mediated the effect of changes in PA/NA on subsequent sleep quality.

RESULTS: Multilevel models showed that changes in sleep quality predicted changes in PA (B=0.08, p<0.001) and NA (B=-0.06, p<0.001), but not the other way around (PA: B=0.03, p=0.70, NA: B=-0.05, p=0.60). Fatigue was found to be a significant mediator of the relationship between sleep quality and PA (Indirect Effect=0.03, p<0.001), and between sleep quality and NA (Indirect Effect=-0.02, p=0.01). Rumination was not investigated because of non-significant associations between PA/NA and sleep quality. The associations were not different for patients and controls.

LIMITATIONS: The analyses were restricted to self-reported sleep quality, and conclusions about causality could not be drawn.

CONCLUSIONS: Improvements in sleep quality predicted improvements in affect the following day, partly mediated by fatigue. Treatment of sleep symptoms would benefit affect in clinical care and beyond.