Sleep: The New Heavyweight in Weight Control?
Abstract & Commentary
By Barbara A. Phillips, MD, MSPH, Professor of Medicine, University of Kentucky; Director, Sleep Disorders Center, Samaritan Hospital, Lexington. Dr. Phillips serves on the speakers bureau for PotomaCME.
Synopsis: Short (< 7 hours/night) sleep duration is associated with increased BMI and increased genetic influences on BMI. Longer sleep duration may reduce genetic influences on body weight.
Source: Watson NF, et al. Sleep duration and body mass index in twins: A gene-environment interaction. Sleep 2012;35:597-603.
The authors of this paper set out to learn whether how long someone sleeps affects his or her body mass index (BMI) as a factor independent from their environment or genetic tendency to put on weight. To do this, they used the University of Washington Twin Registry, which is a community-based sample of twins recruited from the general population of Washington state. All twins were raised together, so environmental and genetic characteristics were matched. The study sample for the current analysis was 2176 individuals from 1088 complete twin pairs (604 monozygotic and 484 dizygotic). Overall, the sample was young (mean 36.6 years), well educated (41% with a college degree or higher), predominantly Caucasian (89%), and female (66%). The most common twin relationship was female monozygotic pairs (38%).
Self-reported usual sleep duration was obtained from responses to the question, "On average, how long do you sleep per night?," reported in hours and minutes. For this study, "normal" sleep duration was considered 7-8.9 hours/night, "short sleep" was < 7 hours/night, and "long sleep" was > 9 hours per night. As a whole, the sample mean sleep duration was 7.2 hours/night (standard deviation = 1.2). In this sample, 317 pairs were short sleepers. The short-sleeping group was 70.0% female, with a mean age of 39 years and a mean sleep duration of 6.06 hours. The normal sleepers were the largest group, made up of 72 pairs, with a mean age of 35.7 years and a mean sleep duration of 7.53 hours. This group was 70% female. There were 47 pairs of long sleepers, whose mean age was 33.3 years and mean sleep duration was 9.3 hours. This group was 73.4% female.
BMI was calculated from self-reported height and weight and was treated as a continuous variable for analytic purposes.
The authors calculated the twin correlation for BMI, separately by zygosity and sleep group. The twin correlations were used to calculate three different quantities: 1) heritability, 2) shared environmental influences, and 3) nonshared environmental influences. Because the long sleepers were slightly younger and more likely to be female, the investigators calculated the twin pair correlations for BMI stratified by sex and age group.
The investigators then fit a more rigorous biometric genetic model for gene-environment interactions, which I frankly don't understand, and certainly cannot make comprehensible to someone else. But to give you a flavor of this part of the discussion, here is an excerpt: "heritability of BMI at any value for sleep duration (x) can be calculated as:
The point was to estimate the relative influences of genes, environment, age, and gender on the effect of sleep duration on BMI. It is probably worthwhile to note at this juncture that this analysis required a fair bit of estimation and that there are very few truly qualified peer reviewers of this work.
Overall, there was a significant main effect of sleep duration on BMI, with longer sleepers having slightly lower BMIs. The investigators also found that males and older adults reported significantly shorter sleep duration, and older adults were heavier.
With regard to the effect of sleep duration on BMI, the investigators found that for the short-sleeping twin pairs, additive genetic influences accounted for 70% of the variance in BMI, whereas shared environmental factors accounted for just 4%. In contrast, for twin pairs averaging ≥ 9 hours of sleep per night, additive genetic factors accounted for just 32% and shared environmental influences accounted for 51%. In addition, a significant negative interaction between genes and sleep duration was noted, indicating that genetic influences on BMI decrease with increasing sleep duration. At the same time, there was a significant positive interaction between shared environmental influences and sleep duration, indicating that shared environmental influences on BMI increase with increasing sleep duration. These findings suggest that genetic influences on BMI are moderated by habitual sleep duration, with genetic influences predominating in short sleepers and environmental influences predominating in long sleepers.
These authors also examined genetic and environmental influences on sleep duration. Of the total variance in sleep duration, 34% was due to additive genetic influences and the remaining 66% to nonshared environmental influences. When looking at the genetic and environmental influences on BMI, they found significant genetic, shared environmental, and nonshared environmental influences on BMI (all P < 0.05) after controlling for the main effects of sleep duration, age, and sex.
Commentary
In our overweight, increasingly health-conscious culture, new information about factors influencing weight receives a lot of attention. For example, the AMA Morning News (May 2, 2012) included the following blurb, "Extra sleep may help overweight people lose weight. NBC Nightly News (5/1, story 11, 0:25, Williams) reported, 'New research out tonight shows that getting a lot of sleep like more than nine hours a night could help overweight people slim down,' while 'sleeping fewer than seven hours a night was associated with higher weight.' Scientists arrived at this conclusion 'by putting sets of twins through different sleep conditions. Researchers think extra rest can work to suppress a gene that is connected to obesity.'"
Let's be clear here. With all due respect to Brian Williams, this was not a randomized, controlled trial, and investigators did not "put sets of twins through different sleep conditions." Sleep duration was self-reported, as it almost always is in these kinds of studies. And, as pointed out in this study, sleep duration is under both genetic and environmental control. But, the frenzy in the lay press resulted in a couple of visits to my sleep medicine clinic by patients who wanted to sleep more so they could lose weight.
This study also adds to our understanding of the fascinating association between habitual sleep duration and health in general. The "right" amount of sleep for humans is a topic of fierce debate. Normal humans sleep 7 to 7.9 hours when left alone to do as they wish.1,2 Sleep duration is both genetically and environmentally influenced, and the heritability of sleep duration is between 31% and 55%.3-5 However, over the past century, habitual sleep duration has dropped 1.5 hours per night, and since 2001 the percentage of U.S. adults getting at least 8 hours of sleep per night on weeknights has fallen from 38% to 27%.6,7
There is evidence that habitual short sleep is associated with obesity.8,9 But much of the data about sleep duration and health outcomes (from death to hypertension) has indicated a U-shaped relationship between sleep duration and outcomes, with both short (typically ≤ 7 hours per night) and long (typically ≥ 9 hours per night) sleep durations associated with increased risk of adverse outcomes.10-13 Indeed, several previous studies have demonstrated higher weights for both long and short sleepers.14,15 However, the population examined in the current study was younger than in many previous studies of this issue, and it is possible that young individuals are more susceptible to the effects of sleep curtailment than are older ones.
What should you tell patients who ask you about this? The main finding of this study is that if you habitually "sleep short" (< 7 hours a night) you are likely to have a higher BMI and to have genetic, rather than environmental factors influencing your propensity to gain weight. Conversely, "long sleepers" (> 9 hours/night) experience greater influence of environmental factors (which presumably includes diet and exercise, although the authors never say so) on BMI. In addition, sleep duration itself is influenced both by genetics and by environment. One interpretation here is that longer sleep may enhance your environmental efforts (e.g., eat less, move more) to lose weight, but cannot make you lose weight in and of itself. Or, as the authors put it, "...the most parsimonious interpretation of our data is that sleep curtailment activates obesity-related genes." But for an individual who comes from a long line of obese people, longer sleep may help to liberate him or her somewhat from whatever genetic tendency he/she has to put on weight. Sadly, at the end of the day, weight is still determined by calories in and calories out.
References
1. Weitzman ED, et al. Timing of REM and stages 3 + 4 sleep during temporal isolation in man. Sleep 1980;2: 391-407.
2. Dijk DJ, et al. Age-related increase in awakenings: Impaired consolidation of nonREM sleep at all circadian phases. Sleep 2001;24:565-577.
3. Partinen M, et al. Genetic and environmental determination of human sleep. Sleep 1983;6:179-185.
4. de Castro JM. The influence of heredity on self-reported sleep patterns in free-living humans. Physiol Behav 2002;76:479-486.
5. Watson NF, et al. A twin study of sleep duration and body mass index. J Clin Sleep Med 2010;6:11-17.
6. Webb W, Agnew H. Are we chronically sleep deprived? Bull Psychon Soc 1975;6:47-48.
7. National Sleep Foundation. Sleep in America Poll. 2002-2011. Washington, DC: National Sleep Foundation.
8. Gangwisch JE, et al. Inadequate sleep as a risk factor for obesity: Analyses of the NHANES I. Sleep 2005;28: 1289-1296.
9. Spiegel K, et al. Brief communication: Sleep curtailment in healthy young men is associated with decreased leptin levels, elevated ghrelin levels, and increased hunger and appetite. Ann Intern Med 2004;141:846-850.
10. Nagai M, et al. Sleep duration as a risk factor for cardiovascular disease-a review of the recent literature. Curr Cardiol Rev 2010;6:54-61.
11. Gangwisch JE, et al. Sleep duration as a risk factor for diabetes incidence in a large U.S. sample. Sleep 2007; 30:1667-1673.
12. Gallicchio L, Kalesan B. Sleep duration and mortality: A systematic review and meta-analysis. J Sleep Res 2009;18:148-158.
13. Singh M, et al. The association between obesity and short sleep duration: A population-based study. J Clin Sleep Med 2005;1:357-363.
14. Taheri S, et al. Short sleep duration is associated with reduced leptin, elevated ghrelin, and increased body mass index. PLoS Med 2004;1:e62.
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