Multiple sclerosis (MS) is an inflammatory autoimmune disorder of the central nervous system affecting over 2.8 million people globally (Walton et al., 2020). Continuous innovation in effective disease-modifying therapies has significantly increased the life expectancy of people living with multiple sclerosis (pwMS) (Vaughn et al., 2019). Thus, it is becoming more common for pwMS to encounter common geriatric conditions; and geriatric conditions like frailty may further reduce the overall health status of pwMS (Zanotto et al., 2022).

Frailty is a clinical syndrome associated with functional decline and lowered reserve that results from cumulative deficits across multiple physiological systems (Pradeep Kumar et al., 2020; Abellan van Kan et al., 2008, 2010; Evans et al., 2010; Fried et al., 2001; Mohler et al., 2014). In older adults, frailty is characterized by high vulnerability to adverse health outcomes, including falls, disability, hospitalization, and mortality (Mohler et al., 2014). However, the clinical implications of frailty in MS are not well understood (Zanotto et al., 2022). Both MS and frailty are independently associated with physical or motor function and cognitive decline (Kelaiditi et al., 2013; Musella et al., 2018). Although motor and cognitive impairments are often regarded as the prime manifestations of MS-related disabilities, there exists a considerable overlap with frailty (Theou et al., 2012).

While the pathogenesis of frailty is typically associated with aging, the overall effect of MS may lead to premature onset of frailty in pwMS (Ayrignac et al., 2021). For instance, Hanlon et al. (2018) analyzed frailty data from 493,737 participants (age: 37–73 years) and concluded that pwMS have a 15-fold higher risk of being frail compared to those without MS (Hanlon et al., 2018). Notably, greater frailty levels are associated with adverse clinical outcomes, such as falls, in pwMS (Zanotto et al., 2022). Despite this, the clinical implications of frailty in the context of MS remain largely unexplored, and there is a critical need to better understand the link between frailty and symptoms of MS.

Sleep is another aspect of health that is adversely impacted by MS, with approximately 70 % of pwMS reporting poor sleep quality (Boe Lunde et al., 2012; Siengsukon et al., 2018). Sleep disturbances are more common in pwMS compared to individuals without MS and negatively affect the quality of life in pwMS (Caminero and Bartolome, 2011; Lunde et al., 2013). Several factors contribute to the poor sleep quality in pwMS including fatigue, pain and spasticity, restless leg syndrome, nocturia, medications, anxiety and depression, and sleep-related breathing difficulties (Brass et al., 2014; Braley and Boudreau, 2016; Braley et al., 2012; Menzin et al., 2013; Veauthier and Paul, 2014).

Sleep and frailty have shown a documented relationship in older adults. Several studies have shown frailty to associate with poor sleep quality (Alqahtani, 2021; Guida et al., 2021; Wai and Yu, 2020), and others have shown it to relate with low sleep efficiency, sleep-disordered breathing, prolonged sleep latency, and excessive daytime sleepiness (Ensrud et al., 2009; Vaz Fragoso et al., 2009; Sun et al., 2020). It is unclear if these associations occur or have unique implications in pwMS.

Due to the high prevalence of both frailty and sleep disturbances in the MS population, it is important to elucidate the association between sleep quality and frailty in pwMS. This could help in identifying potential targets for intervention and improve overall health and quality of life in this population. Therefore, the objective of this study was to examine the association between frailty and sleep in people with both MS and obesity. We hypothesized that frail pwMS with obesity would exhibit poorer sleep quality compared to non-frail pwMS with obesity.