Original Articles
Sleep duration, illumination, and activity patterns in a population sample: effects of gender and ethnicity

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Abstract

Background: Current knowledge of the population’s sleep durations emanates primarily from questionnaires and laboratory studies. Using Actillumes, we investigated whether self-reported sleep durations were indicative of a population decline in sleep duration. We also explored illumination and activity patterns.

Methods: San Diego adults (n = 273, age range: 40–64) were recruited through random telephone calls and were monitored at home while engaging in usual daily routines.

Results: Volunteers slept an average of 6.22 hours and received an average of 554 lux (environmental illumination). The timing of sleep, illumination, and activity occurred at 2:44, 12:57, and 13:43, respectively. Irrespective of ethnicity, age, and time reference, men received greater illumination than did women, but this gender effect was not independent of work status. Women and men exhibited a similar circadian activity profile; however, women exhibited better sleep-wake patterns. Interactions between gender and ethnicity suggested worse sleep-wake patterns among minority men. An age-related decline in activity was found, but no age trend in sleep duration or illumination patterns was observed.

Conclusions: This study showed an objective population decline in sleep duration. Sociodemographic effects should be considered in analyses of sleep-wake patterns and illumination exposures.

Introduction

Current knowledge of the population’s habitual sleep durations and circadian rhythms emanates primarily from questionnaire surveys and laboratory polysomnographic studies Kripke et al 1979, Gallup 1979, Gallup 1995, Schoenborn 1986, Webb and Campbell 1979. Unfortunately, laboratory investigations have shown that subjective questionnaire reports of individuals’ sleep are often inaccurate (Carskadon et al 1976). Moreover, laboratory samples are distorted both by the peculiar laboratory environment and by recruitment methods, which do not yield representative samples of the population.

The past two decades have furnished a converging series of studies that point to a gradual decline in the population’s sleep duration. Williams’ landmark study (Williams et al 1974) found that volunteers slept approximately 7 hours in laboratory polysomnographic recordings. This observation suggested that the average adult might be sleeping less objectively than the proverbial 8 hours, which was popularly believed to be the mode. Indeed, survey data had indicated that ≥ 8 hours was the modal sleep time reported four decades ago (Kripke et al 1979). A Gallup survey (Gallup 1979) also found that 8 hours of sleep was the reported mode. Several years later, the National Health Interview Survey showed that 67% of individuals ranging from 45–64 years of age reportedly slept 7–8 hours at night (Schoenborn 1986).

There was some consternation when the Gallup survey of habitual sleep duration was repeated 15 years later, finding that the modal self-estimated sleep had dropped to 7 hours (Gallup 1995). Notably, in the 1995 survey, 49% of participants reported sleep problems, whereas 36% had reported problems in the landmark survey, “Sleep in America” (Gallup 1991). More recently, the Omnibus Sleep in America Poll (OSAP) observed average reported sleep of 6.57 hours (Gallup 1998).

Objective measures are essential to investigate current sleep durations in the population. Unfortunately, a number of methodological concerns limit the utilization of polysomnography in epidemiological studies Agnew et al 1966, Edinger et al 1997, Hauri and Olmstead 1989, Moses et al 1972, Reynolds et al 1992, Toussaint et al 1995, van Hilten et al 1993. In this investigation, wrist actigraphy (Actillume) was used to monitor sleep and wakefulness patterns, despite some inherent limitations. This methodology was chosen because it facilitates compliance and permits representative recruitment Sadeh et al 1995, Tryon 1991. Furthermore, the Actillume also provides data on activity and illumination patterns in the sample.

Although the influences of the light/dark cycle have been demonstrated in numerous experimental paradigms, little attention has been paid to actual environmental illumination in the population. Evidence from our laboratory has shown relatively little daylight illumination in young and older adults Campbell et al 1988, Cole et al 1995, Okudaira et al 1983, Savides et al 1987. This study expanded on these previous reports by examining illumination exposure in a population sample. Activity, an important circadian behavioral marker, was also considered Aschoff 1969, Moore-Ede et al 1983, Stephan and Nunez 1977. To date, inconsistent findings have been reported regarding the effects of gender and age on activity patterns Brown et al 1995, Jean-Louis et al 1997a, Jean-Louis et al 1997b, Lieberman et al 1987, Renfrew et al 1991, van Hilten et al 1993.

This analysis focuses on gender and ethnicity effects on sleep and wakefulness, illumination, and activity patterns in a sample of adults ranging from 40 to 64 years of age in the city of San Diego. Additionally, we adjusted for effects of age, time reference (i.e., Daylight Saving Time vs. Standard Time), and work status.

Section snippets

Participants and procedure

Wrist activity and illumination data were gathered from volunteers who were recruited by random telephone calls from July 1990 through June 1994 as part of a survey assessing the prevalence of sleep disorders in the city of San Diego. The present analyses were based on a portion of the sample selected for complete data. Telephone calls placed to numbers selected at random were utilized to obtain as representative a sample as possible of San Diego residents (age range: 40–64 years). Consenting

Results

Volunteers (women = 144, men = 129) were all community residents of the city of San Diego (see Table 1). Of the sample, 77% were non-Hispanic Whites and the remaining 23% comprised an aggregate of Hispanics, African Americans, Native Americans, Asians, and Others. Women holding office jobs represented 34% of the sample, and men with office jobs were 41% of the sample, totaling 75% (78.2% for White and 63.0% for minority volunteers).

The remaining 25% of the volunteers indicated that they were

Sleep profile

The present actigraphic investigation demonstrated that the adult population (ages 40–64) sleeps considerably less than was once thought, at least in San Diego. The average sleep recorded for men and women was 373 min, which is somewhat surprising, considering that the current nosologic characterization of insomnia emphasizes sleep durations of less than 6.5 hours (ASDA Diagnostic Classification Steering Committee, 1990). This actigraphic finding corroborates contemporary questionnaire surveys

Acknowledgements

This research was supported by NIA (AG12364 and AG02711) and NHLBI (HL55983 and HL40930). We thank Mary Anne Mowen, William J. Mason, Daniel J. Mullaney, Deborah L. Wingard, Katharine M. Rex, Jeffrey A. Elliott and Terry Allen for their assistance in this study.

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