The prevalence of obesity is increasing worldwide, particularly in the US (1)
. In 2004, more than one-third of American adults were overweight or obese (2)
. Rates of overweight and obesity have also increased in all industrialized countries, although prevalence is still lower than that in the US. shows the prevalence of obesity (body mass index (BMI) >30
) in selected European countries in 1998 and 2001 (3)
. Prevalence rates range from less than 8% to nearly 14%, depending on country and sex, and remarkably the proportion of obese individuals in the general adult population increased over the 3-year follow-up period in all but one country. The epidemic of obesity has not spared children. shows the increase in prevalence of overweight among the US children of ages 2–5, 6–11, and 12–19 years from the early seventies to 2003–2004 (4)
. In each age group, the prevalence of overweight approximately tripled over these three decades.
Figure 1 Prevalence of obesity (BMI>30kg/m2) in nine European countries in 1998 and 2001 (adapted from Ref. (3)).
Figure 2 Prevalence of overweight (defined as above the 95th percentile for age and sex) among the US children of ages 2–5, 6–11, and 12–19 years in 1971–1972, 1976–1980, 1988–1994, 1999–2000, 2001–2002, (more ...)
The causes of this epidemic are not fully explained by the changes in traditional lifestyle factors such as diet (increases in food intake, food portions, and snacking) and decreases in physical activity. It has been recently proposed that shorter habitual sleep times may also be on the causal pathways (5, 6)
. Indeed, one behavior that seems to have developed over the past few decades and has become highly prevalent, particularly among Americans, is sleep curtailment. In 1960, a survey study conducted by the American Cancer Society found modal sleep duration to be 8.0 to 8.9
, while in 1995 the modal category of the survey conducted by the National Sleep Foundation poll had dropped to 7
. Today, more than 30% of adult men and women between the ages of 30 and 64 years report sleeping less than 6
h per night (9)
Very few studies have assessed usual sleep duration using objective measures, rather than self-report. One of the first attempts was a population-based study of 273 volunteers aged 40–60 years in San Diego, California. This study measured sleep by wrist actigraphy in 1990–1994 and found mean sleep duration to be 6.2
. The Sleep Heart Health Study used a single night of in-home polysomnography among 2685 participants with a mean age of 62 years, and the average sleep time was 6.1
h for women and 5.7
h for men (11)
. More recently, an ancillary study to the Coronary Artery Risk Development in Young Adults (CARDIA) study, a prospective multi-center cohort study of the evolution of cardiovascular risk factors begun in 1985, measured sleep in participants aged 38–50 years using wrist actigraphy for three consecutive days in 2003–2004 (12)
. In the CARDIA sleep study, mean (±s.d
.) for sleep duration was 6.1 (1.2) h, and varied between the race–sex groups from 6.7 (0.9) h in white women to 5.1 (1.3) h in African-American men (12)
. Insufficient sleep appears also highly prevalent in the US children. Thus, all three studies that assessed sleep duration objectively in the US adults found very similar means of sleep duration.
Laboratory studies have shown that ‘sleep need,’ defined as the amount of sleep achieved when given 10
h of nocturnal bedtime, does not change substantially across adolescence (10–17 years) and is about 9
. Contrasting with this physiological sleep need of 9
h are self-reported sleep durations collected in a sample representative of the US in a 2006 survey conducted by the US National Sleep Foundation (14)
. As shown in , mean sleep duration was under 9
h in all age groups and decreased steadily from 8.4
h at ages 11–12 years to only 6.9
h at ages 17–18 years. Furthermore the study showed that the decrease in sleep duration was mostly due to later bedtimes, rather than earlier wake up times, and that the respondents were aware of having insufficient sleep.
Figure 3 Self-reported sleep duration (hours) by age for the US children aged 11–18 years (14).
The decrease in average sleep duration in the US has occurred over the past three to four decades, simultaneous with the increase in the prevalence of obesity. In recent years, data from both laboratory and epidemiological studies have accumulated to support the hypothesis that sleep curtailment may have contributed to the increased prevalence of overweight and obesity. Sleep curtailment may affect energy balance and result in weight gain via three distinct pathways: upregulation of appetite, more time to eat, and/or a decrease in energy expenditure. Significant weight gain may in turn result in insulin resistance, a condition that may promote further adiposity. Additionally, up to 20% of overweight and obese individuals suffer from sleep-disordered breathing (SDB), an independent risk factor for insulin resistance (15, 16)
. Indeed, there is a substantial body of evidence to indicate that sleep fragmentation, hypoxia, and low levels of deep non-rapid eye movement (REM) sleep as occur in SDB may all contribute to reduce insulin sensitivity (16–18)
. The present article will, however, focus on sleep loss resulting only from behavioral sleep restriction rather than that from the presence of a sleep disorder.