Testing the contiguity of the sleep and fatigue relationship: a daily diary study

Abstract Study Objectives There is mixed evidence for the relationship between poor sleep and daytime fatigue, and some have suggested that fatigue is simply caused by lack of sleep. Although retrospective measures of insomnia and fatigue tend to correlate, other studies fail to demonstrate a link between objectively disturbed sleep and fatigue. The current study prospectively explored the relationship between sleep and fatigue among those with and without insomnia disorder. Methods Participants meeting Research Diagnostic Criteria for insomnia disorder (n = 33) or normal sleepers (n = 32) completed the Consensus Sleep Diary (CSD) and daily fatigue ratings for 2 weeks. Baseline questionnaires evaluated cognitive factors including unhelpful beliefs about sleep and rumination about fatigue. Hierarchical linear modeling tested the within- and between-participant relationships between sleep quality, total sleep time, and daily fatigue ratings. Mediation analyses tested if cognitive factors mediated the relationship between insomnia and fatigue. Results Self-reported nightly sleep quality significantly predicted subsequent daily fatigue ratings. Total sleep time was a significant predictor of fatigue within, but not between, participants. Unhelpful sleep beliefs and rumination about fatigue mediated the relationship between insomnia and fatigue reporting. Conclusions The results suggest that perception of sleep plays an important role in predicting reports of daytime fatigue. These findings could be used in treatment to help shift the focus away from total sleep times, and instead, focus on challenging maladaptive sleep-related cognitions to change fatigue perception.