Reacción a "Reactions to study linking sleep deprivation in adolescence with increased risk of multiple sclerosis"

This is a case-control study which, although methodologically sound, involves a design that is vulnerable to different biases, such as selection bias. Although it is a population-based study, the percentage of patients with multiple sclerosis (MS) from all over the country who decided to participate and answer the questionnaires that collected the information retrospectively is not recorded. On the other hand, the fact that information on sleep quality from years ago was collected retrospectively is subject to bias for several reasons: first and foremost, patients may provide answers influenced by their current sleep quality or mood, and secondly, even if the diagnosis of MS was made years later, some of the symptoms of the disease, which could include fatigue or sleep quality, may even be considered prodromal. 

Some of the confounding factors are very difficult to control. One aspect that could have been considered is whether the presence of current symptoms that influence the ability to respond negatively correlated with the number of previous hours of sleep reported. In other words, if someone is tired, depressed or very disabled now they may tend to see their quality of life in general, and also their sleep in particular, as worse than it actually was at the time. Looking at these correlations with scales of depression, fatigue or disability might have been interesting in order to rule out a role for these factors. 

This is a study that points to a possible risk factor with some relatively clear methodological limitations, as previously mentioned, and detects a relatively minor magnitude of effect of the risk factor when considering the magnitude of effect of other factors, such as Epstein-Barr virus infection. This makes the practical implications for the day-to-day management of the disease minor, although it may be an interesting idea to explore in future cohort studies. On the other hand, concrete implications would include the need to ensure good quality sleep for children and adolescents, which is a general health recommendation that should in any case be followed regardless of its influence on MS.