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

This is a retrospective retrospective observational epidemiological case-control study that assesses the relationship between exposure or risk factors (short sleep duration and poor sleep quality at ages 15-19 years) and the subsequent development of Multiple Sclerosis (MS). The result is expressed in OR ('odds ratio'), which indicates how much more frequent the exposure to the risk factor was in the cases (MS patients) than in the controls (general population from the same geographical area, Sweden in this case, and of the same age, sex and residential area).  

The OR in this study is 1.4, which means that exposure to these factors (insufficient and poor quality sleep) increases the risk of MS by 40% compared to no exposure. The study design is of good quality with a reliable source of information, as well as the definition of cases and the selection of controls, but with the limitations inherent to this type of study: risk of bias, i.e. systematic errors in the assessment of the results (selection bias, information bias, recall bias). This type of study does not make it possible to establish a causal relationship between the exposure factor(s) (insufficient and poor quality sleep) and the development of the disease (MS), but it does make it possible to establish an association or dependency link between these factors and the development of the disease (MS). The article itself refers to the risk of the aforementioned biases, indicating that they have been minimised and partially controlled, and even to the possibility of reverse causality, i.e. that it is the disease (MS) that causes a quantitative and qualitative sleep deficiency. 

Despite these biases, there is biological plausibility that insufficient and poor quality sleep may increase the risk of MS and contribute along with other known environmental factors (smoking, history of infectious mononucleosis, sun exposure, serum vitamin D levels, adolescent body mass index) to its development. In this disease, it is believed that the set of environmental factors involved (some known and many not yet known) act in an individual genetically predisposed to the development of this disease. There is already evidence in line with this study that shift work, which also affects sleep and circadian rhythms, may increase the risk of MS (Norwegian study), as well as evidence that insufficient and poor quality sleep leads to a series of cellular and molecular changes that alter immune homeostasis and melatonin secretion, all of which have an impact on the development of chronic inflammatory diseases. 

Knowing about this new potential (partially) modifiable environmental risk factor is important to educate the adolescent population in the development of healthy lifestyle habits, including getting enough sleep each day, and trying to ensure that sleep is of good quality and restorative, in order to prevent the development of diseases such as MS. Adolescents and the general public should also be aware that compensating for sleep on holidays or on certain days of the week does not reverse the immune changes that lead to the disease, and therefore oversleeping on other days does not change this risk. Good quality and quantity of sleep is necessary for our immune system to function properly.