Vitamin D is a hormone precursor that is produced either through epidermal exposure to sunlight or dietary intake. Prevalence estimates suggest that approximately 1 billion people worldwide have insufficient vitamin D levels, which may put them at higher risk of fractures and falls, cognitive impairment, certain forms of cancer, Type I and Type II diabetes, and poor cardiovascular outcomes. Insomnia is also related to a number of these outcomes, but to date, there is little to no published research documenting the relationship between vitamin D and insomnia. Data from the University of Washington Twin Registry (UWTR) were used. The final sample contains 245 twin pairs (490 singletons, 164 monozygotic twins pairs, 81 dizygotic twin pairs). Mixed level xii linear regressions were used to control for non independence of pairs. The overall individual-level model found a significant association between vitamin D and Women's Health Initiative Insomnia Rating Scale (WHIIRS) among monozygotic twins. Vitamin D was found to significantly predict self-reported insomnia (b = .0336, p = .012, 95% CI [.0054, .0619]), even after controlling for significant covariates such as perceived stress, sleep hours, number of wakeups, age, and sex. Among dizygotic twins, there was not a significant relationship between vitamin D and insomnia(p > .05). The positive association between vitamin D and insomnia was significant only among monozygotic female twins. Our results do not support our original hypotheses. Our original hypotheses predicted that there would be a significant association between vitamin D and self-reported insomnia. An association in the reverse direction anticipated based on findings of the literature were found in this study. Theoretically driven covariates were added in planned and exploratory analyses. The association between vitamin D and insomnia persisted throughout all analyses among monozygotic female twins. Within-pair analyses suggested that the observed association between vitamin D and insomnia is not causal and is accounted for by shared genetic and/or environmental influences.