Thirty percent of adults experience insomnia, causing distress, impaired functioning, and adverse health outcomes. However, little is known about biomarkers that may inform the understanding of insomnia, including its salience within different racial and ethnic groups. Additionally, knowledge is limited to one important group of individuals who experience insomnia (short sleepers) and factors that may influence effectiveness of their treatment. The aims of this dissertation were to: 1) identify evidence-based biomarkers that have been associated with insomnia in adults through a scoping review, 2) determine if insomnia and discrimination are associated with telomere length (TL) among adults from different racial/ethnic groups, and 3) identify the relationship of psychological distress and sleep-related factors to non-response to CBT-I among short sleepers with insomnia. Twenty-five studies were included in the scoping review. The importance of electroencephalography emerged as a potentially useful biomarker in identifying arousal and inhibition of brain activity associated with insomnia. Conflicting findings regarding relationships of some biomarkers emerged. This review allows researchers to build on what is known and replicate studies to assess reliability. Results from a secondary analysis of the National Institute on Aging’s Health and Retirement Study linked insomnia with shorter telomeres in White individuals. Respondents’ perceived discrimination in everyday life initially strengthened the association between insomnia symptoms and telomere shortening in Black individuals, but the effect was not significant after adjusting for covariates. Further research is needed on insomnia's biological mechanisms and its impact on biomarkers in diverse racial/ethnic groups. Results of a secondary analysis from a Treatment of Insomnia and Depression (TRIAD) study provide evidence that general severity of depression, perceived stress, sleep latency, and sleep reactivity were not associated with treatment response to CBT-I. However, an individual symptom of stress prior to treatment (trouble controlling irritability) was associated with less improvement in response to CBT-I among individuals who slept less than 6 hours per night. Difficulty controlling irritability may reflect a state of heightened stress that precludes effective engagement in CBT-I. Results of this dissertation provide an enhanced understanding of biopsychosocial factors associated with insomnia and its treatment. This new knowledge can inform future research.