How sleep affects body composition

The impact of sleep on health has long been a subject of research. In recent years, science has focused on the interaction between sleep duration and nutritional state, especially since modern lifestyles are causing more and more people to sleep too little.

According to current recommendations, adults should sleep between seven and nine hours a day on average. In everyday life, however, many people do not manage this.

Lack of sleep is felt acutely in the form of fatigue, and reduces physical and mental performance. In the medium and long term, however, it leads to hormonal imbalances and promotes high blood pressure, cardiovascular diseases and diabetes mellitus type 2. 

Eating habits, too, are influenced by a lack of rest. People who sleep too little show altered levels of the messenger substances leptin and ghrelin, which control the feeling of hunger and satiety. Consequently, they tend to eat larger portions and consume more calories due to being awake for longer. Their desire for foods rich in carbohydrates and fat also increases.

Lack of sleep also leads to an increased release of the stress hormone cortisol, which plays a major role in energy metabolism and can thus influence weight and body composition. Under the effect of cortisol, fat is mainly stored on the trunk of the body, while energy is mobilised from the muscles. It also increases the blood sugar level and can promote insulin resistance and the development of diabetes mellitus type 2 over a longer period of time.

In 2017, South Korean scientists investigated the influence of sleep duration on body composition. They interviewed more than 16,000 people who took part in a nationwide nutrition survey between 2008 and 2011 and also had their body composition analysed.

They concluded that people who slept less than seven hours a day had a higher fat mass, were 1.2 times more likely to be obese, and suffered 1.3 times more from abdominal obesity than people who slept the recommended length of time.

Indeed, the researchers were not only able to record the effects of reduced sleep duration on body composition, their study also showed that people who slept longer than 9 hours a day had a reduced percentage of fat-free mass and, above all, muscle.

While the researchers attributed the sleep-associated increase in fat tissue to the changes in hormones and behaviour described above, the cause of the loss of fat-free mass in people who slept longer remained unclear. One of the possible explanations is that prolonged sleep can be a symptom of physical and mental illness. The change in body composition would therefore be the result of reduced physical activity, wasting processes or a deterioration in nutrient uptake, but not the direct result of sleeping for longer.

Lack of sleep has a lasting effect on health. Not only does it affect weight, it also increases the risk of many illnesses. By contrast, sleeping for longer is not unhealthy in itself. However, an increase in the need for sleep could provide an indication of pathological processes.

Even though today's lifestyle is often characterised by a high workload and a multitude of daily tasks, everyday life should be organised in such a way that seven to nine hours’ rest is possible in order to keep the body and mind healthy in the long term.