Genetic and environmental factors involved in sleep quality

Abstract

This doctoral thesis started with the objective of researching the genetic and environmental influences involved in sleep quality. To do so, this work uses several methodologies based on the resources provided by two twin registries. Data available about prevalence in the Spanish population was scarce and not updated. Therefore, Study 1 addressed how sleep quality is among Spanish population and what effect factors such as age, sex or menopausal status have on such quality. To do so, a representative sample of the Spanish population was used. This sample was composed of 2144 participants of the Murcia Twin Registry. This Study found that a high percentage of the Spanish population (38,2%) has poor sleep quality. In addition, women were almost twice as likely as men to have poor sleep quality. Age was another essential factor to explain sleep quality, as sleep quality worsens progressively as age increases. Study 2 addressed the aetiology of the differences in sleep quality. Therefore, the main objective of this study was to disentangle the role of genetic and environmental factors in a Spanish population and test for possible sex differences in the distribution of the variance. Our results showed that genetic factors explain a moderate proportion of the variance (34%), being the rest attributable to non-shared environmental factors. Furthermore, the Study found no sex differences. Study 3 addressed the analysis of the relationship between sleep and body mass index (BMI). Previous studies have shown that there is a strong relationship between sleep quality and BMI. However, the directionality of this relationship is not clear. A co-twin control design which allowed for high levels of control (including genetic factors) was used. A significant relationship between BMI and sleep quality was found in the whole sample. Nevertheless, when discordant twin pairs for sleep quality were selected this relationship appeared weaker and lost significance. On the contrary, when BMI was taken as the outcome variable, the relationship was significant in all conditions, including MZ twins discordant for BMI. These results confirm the strong relationship between sleep quality and BMI, suggesting a causal association and a possible directionality insofar sleep quality would strongly affect BMI, but not the other way around. Study 4 focused on studying the relationship between different characteristics of sleep and antisocial behaviour in children. Specifically, this study addressed the analysis of the genetic and environmental influences underlying the association between sleep characteristics and two dimensions of antisocial behaviour (aggression and rule-breaking). To do so, a sample of children twin pairs from the Michigan State University Twin Registry, composed of 1030 twin pairs (range age 6-12y) was used. Sleep characteristics and the two dimensions of antisocial behaviour were measured trough the CBCL questionnaire. This study confirmed the strong association between sleep and antisocial behaviour in children. All these phenotypes (e.g., six sleep variables and two dimensions of sleep) showed significant genetic influence (65% for aggression, 53% for rule-breaking and between 62% and 89% for sleep variables). However, the pattern of relationships between sleep and antisocial behaviour was different for each of those dimensions. A significant genetic overlap was found between aggression and sleep variables, whereas the genetic correlations between rule-breaking and sleep variables were mostly non-significant and of lower magnitude. On the whole, these four studies add novel information about sleep quality in the Spanish population and specifically deepen in the genetic and environmental influences on this phenotype. These results provide us with further knowledge about sleep quality, highlighting the relevance of genetic factors on its aetiology and its importance for human health and behaviour.