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

The Karolinska Institute group has been working for years on the epidemiology of multiple sclerosis and they do very rigorous studies, as in this case. 

Until now there was no clear evidence that lack of sleep in adolescents was a risk factor. Of course, it is well known that with adolescence the sleep pattern changes and that most adolescents are sleep deprived, but this does not necessarily increase the risk of MS. In this study they show that sleeping less than seven hours slightly increases that risk. As they say, this does not mean that this is the cause, it could be the consequence of already incubating the disease (called the prodrome of the disease in medicine).  

These results help to design public health policies, promoting healthy sleep in adolescents. For people who already have the disease, improving their sleep will help improve their overall health, although it probably won't change how their disease progresses. 

As to how sleep deprivation might cause an increased risk of multiple sclerosis, it could be due to down-regulation of the immune response due to the chronic stress of sleep deprivation. All physiological functions such as sleep and immune response are closely related to each other. 

Like any epidemiological study, it has the limitations that the population studied is limited and that, as the risk factor cannot be changed during the study, in this case lack of sleep, it cannot be established whether this factor is a cause or a consequence or just another factor. Data collection is based on questionnaires and these are always subject to recall and perception biases, even more so in this population so susceptible to social influences. The study is well controlled for confounders such as age, sex, ethnicity, smoking, history of infectious mononucleosis and sun exposure (which determines vitamin D levels, which are very low in Sweden and a risk factor for multiple sclerosis). In a second analysis they adjusted for obesity, a risk factor for poor sleep.