P065 Differentiating sleepiness and fatigue in sleep clinic patients

Abstract Introduction Sleepiness and fatigue are commonly reported symptoms across a range of sleep disorders. Although they differ in underlying mechanisms and carry different treatment implications, these terms are often used interchangeably, making them challenging to distinguish in clinical practice. This study examines whether sleepiness and fatigue represent empirically distinct constructs in individuals seeking treatment for sleep-related concerns. Methods A total of 1550 participants (Mage = 44.56 years, SD = 16.11) recruited from an outpatient sleep clinic completed the Epworth Sleepiness Scale (ESS), Fatigue Severity Scale (FSS), and Insomnia Severity Index (ISI) prior to treatment. Confirmatory factor analysis compared a one-factor model of combined sleepiness and fatigue with a two-factor model representing separate but correlated constructs. Results The two-factor model (CFI = .997; RMSEA = .057; SRMR = .044) provided a better fit to the data than the one-factor model (CFI = .953; RMSEA = .239; SRMR = .208). In the preferred model, sleepiness and fatigue latent factors were moderately correlated (.35). Structural equation modelling examined the predictive utility of the two latent factors for insomnia severity. Fatigue independently predicted greater insomnia severity (β = .60, p<.001), whereas sleepiness independently predicted lower insomnia severity (β = –.09, p=.001). Discussion This study revealed that, among treatment-seeking individuals, fatigue and sleepiness emerge as distinct but related experiences with unique relevance for difficulties initiating and maintaining sleep. These findings highlight the importance of assessing both symptom domains in clinical settings, as well as the potential for targeted intervention.