A small study from the United States has suggested that getting too little sleep might prevent dieters from losing as much body fat as they would have had they slept properly.

When individuals slept for five and a half hours a night, they lost half as much body fat as when they were allowed to sleep for eight and a half hours a night and 50% more lean body mass. These results highlight the importance of human sleep for the loss of body fat when dieting.

Many people today are overweight or obese – the prevalence of obesity in England is thought to be 33% and 28% for men and women, respectively. Furthermore, 42% of men and 30% of women are expected to be overweight in 2010.

Losing weight by dieting is one sensible way of addressing this epidemic and thus reducing the poor health associated with being overweight or obese. However, emerging data suggest that insufficient sleep might affect the response of our hormones to reduced food intake and negatively influence the metabolic effects of eating less.

A total of 12 overweight, non-smoking, middle aged (range 35 to 49 years) individuals were enrolled in this study, of whom 10 (three women and seven men) took part. Alarmingly, one woman was withdrawn from the study because the combination of calorie and sleep restriction caused heart palpitations, whereas another woman dropped out for reasons unrelated to the study.

Participants spent two 14 day stints in the University of Chicago sleep laboratory, with three months inbetween to recover. To start with six were allowed 8.5 hours of sleep a night, while the other four were only allowed 5.5 hours. Then the groups switched over so that those who had been enjoying 8.5 hours a sleep a night were only allowed 5.5, and vice versa.

Before and after each study period, fasting body weight, adiposity, fat-free body mass, and the levels of various hormones related to metabolism were measured. Participants ate their normal diet during each 14-day intervention period, but its calorie content was 10% than what they would usually eat.

Interestingly, both the sleep situations – 5.5 hours a night and 8.5 hours a night – were associated with about 3kg of weight loss. However, when people were sleeping 8.5 hours a night, roughly half of the weight loss (1.4kg) was down to reduction in fat, compared with only about a quarter (0.6kg) when people had 5.5 hours of sleep (55% less fat loss in the sleep reduction group). When sleep was restricted to 5.5 hours a night, people lost more fat-free body mass instead, which is largely made up of muscle and water (60% increase in fat-free weight loss).

People were also hungrier and had higher concentrations of the hormone ghrelin when they only got 5.5 hours of sleep a night. Ghrelin has been shown to reduce energy expenditure, stimulate hunger and food intake, and promote retention of fat. People also had a lower resting metabolic rate during the 5.5 hours a night study period – all the better to ensure the finite amount of energy available would stretch to cover the longer waking hours.

The authors suggest that in the sleep deprived state, the body concentrates increasing amounts glucose in fat tissue to support the more prolonged energy needs of the glucose hungry brain. When energy intake is reduced in this situation, the body “holds on” to fat stores to make sure enough energy is available for the drawn out waking hours and instead uses energy stored in muscle – the fat-free body mass.

The authors also suggest that people trying to lose weight who don’t get enough sleep could find it harder to stick to their diet and might be more susceptible to piling on the pounds if they do slack off. “The enhanced metabolic, neuroendocrine, and behavioral compensation in the form of increased hunger and reduced energy expenditure that develop in response to combined caloric and sleep restriction can disrupt adherence to a lower-energy diet and promote efficient weight regain once it is discontinued,” they write.

Writing in an editorial on the topic, Shahrad Taheri of the University of Birmingham, UK, and Emmanuel Mignot from Stanford Sleep Medicine Center in California suggest that sleep should be included as part of the lifestyle package for weight loss, which traditionally has focused on diet and exercise instead.

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Nedeltcheva AV et al. (2010) Insufficient sleep undermines dietary efforts to reduce adiposity. Annals of Internal Medicine 153 (7): 435-41. PMID: 20921542