Clinical ReviewSecular trends in adult sleep duration: A systematic review
Introduction
Despite the enormous growth of sleep research over the past four decades, little is known about secular trends in adult sleep. Concerns over decreasing sleep duration have gained prominence as progressively more research points to short sleep as a health risk factor. Self-reported habitual sleep duration of less than 6 h per day has been reported to predict premature mortality1, 2 and cardiovascular risk.3 Insufficient sleep is associated with lost productivity and increased risk of accidents4 and injuries.5 There are also suggestions that deficits in sleep duration are contributing to the current obesity epidemic.6 Given these findings, significant decreases in population sleep may have important implications for public health.
Although many have remarked that sleep duration has been declining in recent years7, 8, 9, 10 or have suggested that modern society is increasingly sleep-deprived,11, 12, 13, 14, 15 empirical evidence for this assertion is limited. Support for the trend appears to stem from public polls, the results of which are popularised by the mass media.16 For instance, the National Sleep Foundation poll reported that up to 27% of Americans slept less than 6 h in 2011,17 compared to only 12% in 1998.18 The US Gallup polls also show a similar trend with modal sleep of 8 h in 1979 compared to 6.6 h in 1998.19 Earlier data from the American Cancer Society Survey indicated that sleep duration had fallen 1 h from 1959 to the mid-1980s.20 More recently, online surveys of Australians revealed that average sleep in 2010 was 7 h21 compared to 8 h a decade earlier.22 While suggestive, data from these types of studies are clearly limited by the potential for selection, information, and response biases.
The purported reductions in sleep duration have been attributed to the contemporary 24-h lifestyle, facilitated by the rise of communication technology.10 Cross-sectional studies have shown that longer working hours,23 commute times,23, 24 and television-viewing24 are associated with shorter sleep duration in adult populations.
A secular decline in sleep duration would be consistent with sporadic findings in the medical literature that university students sleep 30 min less in the 1980s compared to the 1970s,25 that daytime fatigue in American men has increased from the 1930s to the 1980s,20 and that sleeping difficulties have been increasing in Sao Paolo, Brazil.26 The generalisability of some of these findings is clearly limited and data from more representative samples are required to determine whether secular declines in sleep duration have occurred. This is particularly important since reports from the social science literature indicate in contrast that sleep times have been relatively constant27 and may have even increased28 over time.
The aim of the current paper was therefore to test the hypothesis that there has been a secular decline in sleep duration over recent years through a systematic review of the literature.
Section snippets
Study selection
We searched five electronic databases for articles pertaining to secular changes in sleep duration on 14th March 2011. The search was subsequently updated on 23rd May 2011. The general search strategy involved sleep duration keywords intersected with time trend keywords and restricted where possible to adult human populations and original articles or conference proceedings. The exact search strategy differed slightly depending on the database and its capabilities (Table 1). Each database was
Characteristics of included studies
The 12 included studies provided results on sleep in 15 countries, 13 of which are OECD countries: Austria,49 Belgium,47 Britain,34 Bulgaria,47 Canada,50 Finland,43 France,47 Germany,47 Japan,47 Korea,40 the Netherlands,38 Poland,47 Russia,35 Sweden,48 and the United States.*42, *44, *46, *47 All of the studies analysed repeat cross-sectional data.48 The data spanned the 1960s to mid-2000s but the number of cross-sections differed by country.
The populations investigated were mainly
Main findings
Despite claims in lay and scientific publications that adult sleep duration has declined over the past four decades we found no consistent evidence this has occurred. Sleep duration has decreased in six countries,*35, *43, *47, *49 increased in seven countries,*34, *38, *40, *47, *50 and there were mixed results for two.*42, *44, *46, *47, *48
Variation between countries
There appears to be no consistent pattern to the changes in sleep duration across countries. Variations in the demographic structure of the population,
Conclusion
Our review of the extant literature shows a mixed pattern with both secular decreases and increases in sleep duration across 15 developed countries. These changes are country-dependent and may reflect social or cultural patterns in sleep duration, differing demographic profiles between these countries, or differences in measurement. The repeated claims that a range of contemporary social factors are impinging upon adult sleep in a globally consistent fashion may reflect a publication or
Acknowledgements
NG has received sleep related research funding from the National Medical Research Council of Australia and Servier Laboratories, and honoraria for advisory group meetings and lectures from CSL Laboratories and SanofiAventis. YSB was supported by a PhD scholarship from the Centre for the Integrated Research and Understanding of Sleep (CIRUS).
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The most important references are denoted by an asterisk.