It is assumed by many that sedentary behaviour is the cause of obesity in humans. Reductions in physical activity are often blamed for the increasing rates of obesity seen in Western nations. In particular, the watching of television and the use of video games in adults and children, respectively, are often cited as the cause of excessive weight gain due to the inactivity associated with such endeavours. This is interesting, because to date no clinical trials have measured weight gain as a result of sitting subjects in front of the television. The fact that sedentary behaviour is often found to be associated with obesity is interesting, but it is not possible to ascribe the cause and effect of this relationship. Obesity is inversely associated with length of sleep. Which contradicts the viewpoint that television causes obesity, because if sedentary behaviour was the cause of weight gain, sleeping more should result in more weight gain rather than less. There is clearly more to these relationships that simple energy conservation causing weight gain.
Although clinical trials are lacking, the association between obesity and sleep duration or television watching has been extensively studied using observational studiues. For example in opne study, researchers1 investigated the sleep duration and television viewing habits of 4452 adolescent subjects through a questionnaire, and determined the relationship to body mass index and body fatness as well as blood pressure. As has been shown previously (here), short-sleep duration was associated with increased body mass index and increase body fat skin folds. The authors calculated that each hour of sleep reduces body mass index by around 0.16 kg/m2. Deprivation of sleep is a form of stress and therefore short sleep deprivation is able to increase the release of cortisol. Cortisol raises blood sugar and contributes to insulin resistance which can lead to accumulation of abdominal fat. This explains the association between short sleep duration and body mass index and body fatness seen in this study.
In addition, sleep deprivation was also associated with increased systolic blood pressure. Development of abdominal obesity is a risk factor for high blood pressure because the inflammation associated with high adipose fat levels causes endothelial dysfunction and this decreases the dilatation of blood vessels in response to flow. The authors also showed that increased body mass index and increased body fat skinfold measurements were associated with television viewing, which could be interpreted as showing that sedentary behaviour results in the accumulation of excessive body fat levels due to decreased physical activity. However, obesity is a metabolic dysfunction that results in the inability to utilise energy correctly. As a result obese individuals are unable to generate sufficient work output in their skeletal muscle without experiencing significant premature fatigue, when compared to normal weight individuals. They are therefore more likely to display sedentary behaviours such as watching television.
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