Interpreting Wrist Actigraphic Indices of Sleep in Epidemiologic Studies of the Elderly: The Study of Osteoporotic Fractures

SLEEP DISRUPTION IS AN EXTREMELY COMMON PROBLEM IN THE ELDERLY POPULATION ASSOCIATED WITH DECREMENTS IN QUALITY OF LIFE AND medical health status.1–8 Epidemiologic data indicate that 12% to 54% of older individuals report symptoms of sleep difficulties on a consistent basis, with a higher percentage of self-reported sleep disruption in women compared to their male counterparts.6,7,9 For example, in a study of nearly 2400 older individuals, the prevalence of insomnia was 54% in women and 36% in men.7 Subjective symptoms of sleep problems have been associated with a wide variety of adverse health outcomes. In the Study of Osteoporotic Fractures (SOF), women with self-reported increased napping and longer sleep had a greater risk of falls and fractures.10 These findings are further corroborated by results from a separate study demonstrating an association between disrupted sleep and falls reporting an overall higher risk of falls in women compared to men with sleep difficulties even after consideration of confounding factors.11 Also, results from the National Sleep Foundation 2003 survey highlight increased napping in the elderly which is associated with the diagnosis of depression.12 Prospective, population-based data also support a relationship between daytime naps in the elderly with increased mortality.13 Self reported sleep problems may represent manifestations of underlying sleep disorders such as sleep-disordered breathing (SDB) and/or periodic limb movement disorder (PLMD). These data underscore the importance of understanding the basis for poor sleep in older individuals, in particular elderly women, as they are at increased risk of adverse outcomes including increased mortality.5,7,8,10–22 In epidemiological studies, objective measures of poor sleep have been quantified largely by use of actigraphy, which employs a watch-like device typically worn on the wrist used to record a digitally integrated measurement of gross motor activity as indicators of sleep wake parameters. Compared to standard single-night polysomnography, actigraphy has the attributes of decreased obtrusiveness and minimal individual burden as well as the ability to record continuously over several weeks or longer providing data over the course of multiple sleep periods. Actigraphy thereby takes into consideration night-to-night variability of collected sleep measures. Impaired sleep determined by wrist actigraphy has been reported to be associated with falls, fractures, impaired cognition, and increased mortality.10,15,23,24 In the SOF cohort, women with short sleep (total sleep time [TST] ≤5 h and those with reduced sleep efficiency (SE, defined as the amount of sleep given the amount of time in bed) as defined by wrist actigraphy, experienced an increased mortality risk compared to those with longer sleep or higher SE.24 Actigraphically defined reduced SE, prolonged sleep latency, increased wakefulness after sleep onset and increase in naps have been shown to be associated with impaired cognition in a cohort of elderly females.15 Despite the growing use of actigraphy in epidemiologic studies for characterizing and quantifying sleep problems, the bases for actigraphically ascertained poor sleep are uncertain; and may reflect primary sleep disorders such as SDB and/or PLMD. It is also unclear whether individuals with actigraphically determined sleep disruption have underlying primary sleep disorders or other comorbid factors such as depression that predispose to or may account for reduced quality of sleep. Poor sleep determined by wrist actigraphy defined as reduced SE or reduced TST may in essence be a non-specific “marker” for other comorbidity and not represent specific sleep disorders. We chose to capitalize on the assembly of both standardized polysomnography and wrist actigraphy data from the SOF study, a longitudinal cohort of elderly women, to better understand whether actigraphic measures of reduced SE and decreased TST are associated with polysomnography-determined SDB and/or PLMD symptoms in older women.