Sleep duration and incident frailty: The Rural Frailty Study

While it is true that aging is associated with changes in sleep, such as decreased total sleep time, lack of deep sleep, and waking up more at night, such changes are not a normal part of the aging process [1]. Optimal sleep, even in older age, is of key importance for overall health and wellbeing. In fact, evidence shows that older adults (OA) who experience sleeping problems can have a low quality of life, cognitive decline, depression, difficulty doing basic daily activities, as well as an increased risk of falls, cardiovascular disease, and even death [1]. Frailty increases susceptibility to adverse effects like disabilities and death [2], and tends to appear in later ages. Results from a meta-analysis indicate that after an average follow-up time of 3 years, 13.6% of OA not considered frail at baseline become frail [3].

Having low quality sleep and sleeping few hours, or too many, has serious physiological effects on the brain and profound biological stress on the cells, organs, and the body’s main regulatory systems. Therefore, it is plausible that certain sleep characteristics, like hours of sleep or sleeping disorders, could be related to incident frailty [4]. In fact, recent studies have linked sleep to frailty, including sleep complaints [5], sleeping less than the number of recommended hours [6‐8] and/or sleeping too much [7‐12], insomnia [6], or low quality of sleep [9, 12, 13]. Nonetheless, most studies are cross sectional and until now, the results have not been conclusive. The purpose of this study was to analyze the relationship between sleep duration, the presence of sleep complaints, and the risk of frailty in community dwelling older adults.

The average age of the OA in the second round was 76.2 ± 3.2 years and 55.3% were women. Accumulated incidence of frailty in the period 2013–2018 was 30.4% (n = 94). Sociodemographic and health characteristics of the OA by frailty status in 2018 are shown in Table 1. At baseline, a sleep duration of 7–8 h was reported by 35.6% of participants. The proportion of OA categorized with 5 h or less was 13.3 and 38.5% reported a sleep duration of 9 h or more. Characteristics of the OA by sleep duration are shown in Table 2. Diabetes seemed to be associated with sleep duration (P = 0.019); OA with long sleep or shorter duration were more prone to self-report diabetes. Sleep duration was not associated with other health conditions or demographic characteristics.

The group of OA who became frail, in a follow up period of 4.4 years, tended to sleep more or less than the recommended hours than OA who were pre-frail or robust (P < 0.01). Furthermore, the presence of sleep complaints was marginally greater in OA with incident frailty (P = 0.09). Differences in sociodemographic variables by incident frailty were observed. The percentage of women was higher in the group of frail OA than in the pre-fail and robust group (P < 0.05). The OA with frailty at follow up were older (P < 0.01) and less likely to have paid work (P < 0.01). In addition, the OA who developed frailty had more health problems than those who were pre-frail or robust, reporting more memory complaints (P < 0.01), limitations in ADL (P < 0.01) and IADL (P < 0.01), poorer physical functioning (P < 0.01), and a marginally higher frequency of hypertension (P = 0.09) (Table 1).

The unadjusted Poisson regression model showed a strong relationship between sleep duration and increased risk of being frail at follow up (Model 1, Table 3), which persisted when sociodemographic covariates were controlled for (Model 2, Table 3) and in the completely adjusted model (Model 3, Table 3). When compared with OA who reported sleeping 7–8 h, the relative risk (RR) of being frail at follow up was 1.8 among those who reported sleeping 5 h or less (95% CI 1.04–3.11) and 1.69 for those who habitually slept 9 h or more (95% CI 1.10–2.58).

Sleep complaints were marginally associated with incidence of frailty in the unadjusted model (RR 1.40, 95% CI 0.96–2.04, P = 0.08). When controlling for age, sex, ethnicity, education level, work, and marital status, the relative risk was 1.42 (95% 0.94–2.15, P = 0.09). Lastly, in the model adjusted for sociodemographic and health variables, sleep complaints were not associated with incidence of frailty (RR 1.41, 95% CI 0.94–2.12, P = 0.10).

This study shows that sleep duration is associated with the incidence of frailty in OA (70 years and older) who live in rural communities in Mexico. In particular, it found that OA who sleep little (5 h or less) or too much (9 h or more) have an increased risk of incident frailty compared to those who sleep 7–8 h a day.

Consistent with our results, one study demonstrated that sleeping 6 h or less, or sleeping 9 h or more, is associated with a greater possibility of becoming frail [7]. Other investigations have reported that only short sleep duration (fewer than 6 h) [6, 8] or only long duration, increase the risk of frailty [8‐12]. Nonetheless, these studies are based on cross sectional analyses. A longitudinal study with 3.4 years of follow up documented the negative effects of low quality sleep and nocturnal hypoxemia on frailty and mortality, but other indicators like sleep duration and latency did not have any effect on frailty or mortality [19]. To our best knowledge, this is the first prospective study that reports that both sleeping too little or too much are related to incidence of frailty at 4.4 years of follow up.

Although a cross sectional association was previously reported between sleep complaints and frailty in the same cohort of subjects [5], this relationship was not confirmed in the longitudinal analysis. It is likely that the longitudinal effect of sleep complaints is mitigated through other mechanisms, specifically due to the presence of unfavorable health conditions. In our study, we observed that by adjusting the variables related to health, the effect of sleep complaints on incidence of frailty was not maintained. Future research should use more objective and exhaustive measures to study sleep disorders and their association with incidence of frailty [20].

The mechanisms that underlie the association between short sleep duration and frailty involve disruptions in neuroendocrine regulation. Reductions in testosterone levels have been observed in individuals who report sleeping few hours, as well as chronic inflammation, greater oxidative stress levels, and growth hormone secretion imbalances, among others [4, 21]. Although the mechanisms behind long sleep duration are still not well understood, it is possible that its relationship with frailty can be explained as follows: Having biologically longer nights means longer periods with elevated melatonin and cortisol levels, as well as a lower body temperature [22], which can affect the immune system [23] and increase the likelihood of becoming frail. Morgan et al. (2018) state that OA who sleep a lot have low daily physical activity levels, reduced muscle strength, and slow walking speed [24].

In this work we studied frailty according to Fried’s phenotype model; however, other models have been developed; the deficit accumulation model is a widely recognized, it views frailty in terms of number of deficits that a person presents in various areas ranging from the physical (symptoms and diseases), as well as psychological and social domains. While Fried’s phenotype model predominates in the study of the link between sleep and frailty [20]; there are few studies that had employed the deficit approach. Balomenos et al. [12] found a cross-sectional relationship between sleep quality and frailty (frailty with three different measures: The Frailty Index (FI); the Tilburg Frailty Indicator (TFI), and the Groningen Frailty Indicator (GFI)) [12]. Del Brutto et al. [13] also documented an association between poor sleep quality and frailty using Edmonton Frail Scale [13]. Sleep duration showed a less clear association across frailty measures [12]. In the men sub-sample, long sleepers were more prone to be frail when it was evaluated by FI; sleep duration and GFI were only marginally associated [12, 25]. Thus, future research should explore sleep disorders in the context of different frailty approaches.

There is a notion that frailty prevention is possible. Majority of research has tested frailty interventions focusing on nutritional supplements, training programs and a combination of both [26]. There has been limited studies testing the effect of sleep interventions to prevent frailty. Our results highlight a potential for designing strategies to reduce frailty incidence by promoting healthy sleep hours, thus, adopting interventions focusing on sleep might potentially avoid progressing to frailty.

This study has various strengths. As far as we know, this is one of the first longitudinal studies with OA participants from Latin America. Frailty was evaluated according to international recommendations of Fried et al. and the study’s longitudinal design allows the relationship between sleep duration and incidence of frailty to be analyzed prospectively. However, the study also has some limitations. Although evaluating sleep duration with self-reports is a common practice in epidemiological community based studies, in the future more objective measures can be used like actigraphy. One important limitations of our study are that measures of sleep quality or sleep disorders were not evaluated, such as: naps during the day, fragmented sleep, the presence of excessive drowsiness, sleep apnea, among other alterations, whose inclusion may have strengthened our findings. Sleeping long duration, for example, could be associated with daytime sleep or with the presence of fragmented sleep. In particular, sleep fragmentation has been associated with decreased energy and increased function limitations [27]. At the end, healthy sleep is multidimensional, beyond the duration of sleep, as a recent study showed that regular snoring was associated with a lower chance of achieving healthy aging [28]. Heavy snoring may also lead to repeated arousals and fragmented sleep that disrupt sleep quality.

This research adds to the scarce number of studies that have analyzed the relationship between sleep and frailty. Our results show that both short sleep duration and long duration are associated with incidence of frailty. More longitudinal studies are needed to clarify the role of sleep characteristics in frailty in older age.