Wednesday May 2 2012
Can you sleep yourself thin?
If you want to lose weight you should “get more sleep”, says the Daily Mail. The newspaper says that going on a ‘sleep diet’ of extra shuteye may be a more relaxing way to shed pounds than counting calories or going to the gym.
While the idea of sleeping yourself thin may seem like a dream come true, these claims are rather unfortunately not wholly supported by the research behind them. They are based on a study that explored whether the amount of sleep someone gets alters the way their genetics influence their body mass index (BMI). To do so, researchers looked at the sleep patterns of twins, both genetically identical and non-identical, so that they could establish how much genetics influenced BMI, and how much sleep modified the relationship.
Researchers found that regularly getting fewer hours of sleep was associated with slightly increased BMI, with sleeping less than seven hours a night associated with genetic factors having an increased influence over BMI. Conversely, sleeping nine hours or more per night was associated with having slightly lower BMI and genes having a reduced influence over BMI.
The study is limited in some respects, including the fact that participants reported their own height, weight and sleep duration, making the results potentially less reliable. The study also assessed sleep and BMI at the same time, making it difficult to determine whether sleep could influence BMI or vice versa. Most importantly, this study has not looked at whether changing our sleep patterns can actually go on to influence our BMIs. It merely suggests that in a population that sleeps less, genetic factors may have a greater influence on BMI.
Where did the story come from?
The study was carried out by researchers from the University of Washington, University of Texas and the University of Pennsylvania. The study was funded by the US National Institutes for Health and the University of Washington. The study was published in the peer-reviewed journal Sleep.
Media reports of this complex study tended to be overly simplistic. In particular, the Daily Mail’s advice that “grabbing some extra shuteye” may be a more relaxing way to slim than “arduous gym sessions and endless calorie counting” is not supported by this study, which did not compare diet and exercise against sleep as methods of weight loss.
What kind of research was this?
This was a study that used a sample of over 1,000 pairs of US twins to examine whether how long people sleep interacts with genetic influences on body weight, as measured by body mass index (BMI). The study is a continuation of an earlier research on a subset of the same sample of twins, which reportedly found that short sleep was associated with higher BMI. The current study did not primarily focus on whether sleep duration is associated with BMI, but whether the amount of sleep was related to the amount of influence their genetics had on their BMI.
The authors report that over the past century, sleep duration has dropped by 1.5 hours a night and since 2001 the percentage of US adults getting at least 8 hours sleep a night has fallen from 38% to 27%. They point out that sleep duration has declined and rates of obesity (defined as a BMI of 30 or more) have increased, and say that evidence is mounting that chronically reduced sleep times are associated with obesity.
While normal sleep need in humans is considered to be between 7 and 8 hours, previous research has suggested that genetics play a substantial role in determining the amount of sleep we need.
Scientists often turn to twins to study how much influence genetics and environment have on variations in characteristics such as sleep duration or BMI. Identical twins inherit the same genetic make-up, while non-identical twins only share about half of their DNA. This type of twin study examines how similar identical twins are and compares this to how similar non-identical twins are for the same characteristic: if a characteristic is largely determined by genetics then identical twins would be expected to be much more similar than non-identical twins. Conversely, if genetics have no influence on a characteristic, then identical and non-identical twins would be likely to share or vary in their characteristics to a similar extent. Studies of this type use computer modelling to estimate the contribution of genetics and environment to the variation seen in the characteristic in the study population.
What did the research involve?
Researchers sourced 1,088 pairs of twins from a US twin registry, with 604 pairs being identical (i.e. they originated from the same fertilised egg). The rest were non-identical (they developed from separate fertilised eggs). Two-thirds of the twins were women, the sample was predominantly white, and the average age was 36.6 years.
The authors based their analysis on a survey in which the participants’ were asked how long they slept at night on average, and reported their height and weight, as well as age, sex and race. Researchers used the data they had to calculate the participants’ BMIs.
From this data researchers divided the sample into three groups according to average sleep duration:
- short sleep – average sleep less than 7 hours a night
- normal sleep – average sleep 7 to 8.9 hours a night
- long sleep – 9 hours of sleep per night or more
The researchers then used computer modelling to compare the identical and non-identical twins and calculate how much of the variability in BMI seen between the twins was down to genetics (called ‘heritability’). They looked at whether the ‘heritability’ of BMI differed among the groups who slept for different durations.
What were the basic results?
According to participants’ self-reported information, average BMI was 25.3kg/m2 and the average sleep per night was 7.2 hours. Overall, those who slept for longer were reported to have slightly lower BMIs.
The researchers found that sleep duration significantly modified the extent to which genetic factors contributed to BMI. Among the group with an average sleep duration of less than 7 hours, genetic factors accounted for 70% of the variability in BMI seen. Among those with an average sleep duration of 9 hours or more, genetic factors accounted for only 32% of the variability in BMI seen.
How did the researchers interpret the results?
The researchers concluded that shorter sleep time is associated with both increased BMI and genetics having a greater influence on BMI. They also say that longer sleep time may suppress genetic influences on BMI.
The authors suggest that future research may benefit from considering the role of sleep duration when searching for specific genetic factors involved in controlling BMI.
This research suggests that the extent to which our genetics influence our BMI varies according to how long we sleep. For traits such as weight and BMI, both genetic and environmental factors are generally thought to play a role, and there is some evidence suggesting that genetic and environmental factors may also be able to interact with each other rather than just exist independently. The current study suggests such a interaction between sleep and genetics and their influence on BMI, although further research will need to confirm it.
The study does have some limitations, including its reliance on self-reported information from participants on height, weight and sleep, which may make the results less reliable, particularly as the observed differences in BMI were quite small. In addition, the study looked at sleep and BMI at the same point in time, meaning that it is difficult to disentangle whether sleep could have influenced BMI, or vice versa. Also, the study did not assess the specific role of other factors that could influence sleep and BMI, such as diet and physical activity. Finally, most participants were predominantly younger, white women, and the study was carried out in the US. Whether similar results would be found in the wider population is uncertain, and results may vary in different countries.
An important point to note is that although the news reporting of this study has focused on the potential for us to lose weight by sleeping more, this study has not looked at whether changing your sleep patterns can affect your BMI.
It is recognised that adequate sleep is important to health in many ways, although whether sleep is a factor in rising obesity rates is not proven by this study.