To Örebro University

Background: Difficulties initiating and maintaining sleep (DIMS) are frequent features of autism, yet little is known about why these conditions co-occur. One possibility is that they share etiological factors, yet this hypothesis remains to be tested using quantitative genetic designs. We thus investigated etiological links between autism and DIMS using familial co-aggregation and twin methods.

Methods: Twins, siblings, half-siblings, and cousins of 50,097 individuals with autism were identified from Swedish population registries. Their risk of DIMS, defined through diagnoses of insomnia and/or melatonin prescriptions, was then estimated. Twin analyses conducted on 15,279 child and adolescent twin pairs investigated etiological links between DIMS and ASD.

Results: 22.8% of autistic individuals had DIMS. Monozygotic co-twins of individuals with autism were most at risk of DIMS compared to the reference group (OR = 6.6 [2.5-17.4]), followed by dizygotic co-twins (OR = 2.6 [1.5-4.5]) and full siblings (OR = 2.5 [2.4-2.6]). Half-siblings and cousins of individuals with autism were least likely to have DIMS relative to the reference group (OR range = 1.3-1.5). Twin analyses estimated a correlation of 0.57 (0.53-0.61) between autism and DIMS, with a genetic correlation of 0.62 (0.60-0.68). These overlapping genetic factors explained 94% of the covariance between these conditions. Autistic traits also showed genetic overlap with DIMS.

Conclusions: Our results suggest that shared genetic mechanisms underlie autism and DIMS, which may lead them to co-occur. Untangling the etiological overlap between these conditions has potential to assist in understanding the etiology of each condition, as well as their associated outcomes.