Association between vulnerability to stress-related insomnia and insomnia severitymay be moderated by 5HTTLPR genotype

Introduction: Serotonin is implicated in the control of sleep-wake be- haviour. A functional polymorphism in the serotonin transporter gene (5HTTLPR) has previously been associated with insomnia, although results are mixed. The present study aimed to determine whether al- lelic variation in 5HTTLPR was associated with insomnia severity in the general population, and whether the association between vulner- ability to stress-related insomnia and insomnia severity was moderated by 5HTTLPR genotype. Methods: DNA from buccal swabs was genotyped for the 5HTTLPR polymorphism using polymerase chain reaction from 95 individuals from the general population. Of these, 77 also provided data on in- somnia severity (Insomnia Severity Index) and vulnerability to stress- related insomnia (Ford Insomnia Response to Stress Test) (mean age: 25.79 years [SD = 9.22 years]; 76.6% female). High and low vulnerabil- ity groups were determined based on a median split of FIRST scores. Results: There was no main effect of genotype on insomnia sever- ity. However, the interaction between 5HTTLPR genotype and vul- nerability to stress-related insomnia on insomnia severity showed a trend towards significance (F(1,73) = 2.93, p = 0.09). Follow-up t-tests revealed that individuals homozygous for the ‘short’ allele who also were categorised as high vulnerability to stress-related insomnia dem- onstrated greater insomnia severity compared to those categorised as low vulnerability to stress-related insomnia (mean insomnia severity scores for low vulnerability SS genotypes: 4.09 [SD = 4.66] and high vulnerability SS genotypes: 11.07 [SD = 6.04], t(23) = −3.16, p.00); whereas there were no significant differences in insomnia severity for individuals carrying at least one ‘long’ allele categorised as high or low vulnerability to stress-related insomnia (mean insomnia severity scores for low vulnerability SL+LL genotypes: 6.20 [SD = 5.56] and high vulnerability SL+LL genotypes: 8.86 [SD = 5.30], t(50) = −1.56, p.13). Conclusion: The association between vulnerability to stress-related insomnia and insomnia severity appears to be moderated by 5HTTLPR genotype. Individuals carrying at least one ‘long’ allele appear to be protected from experiencing more severe insomnia despite possessing a trait-like vulnerability to insomnia.