OBJECTIVE: Comorbidity and subdiagnostic symptoms are understudied for sleep and circadian problems. We evaluated 1) impairment associated with (a) number of sleep and circadian problems and (b) diagnostic threshold (full diagnosis vs. subdiagnostic symptoms), and 2) Transdiagnostic Sleep and Circadian Intervention (TranS-C) outcomes for participants with specific sleep and circadian problems. METHOD: Community participants (N = 121) with serious mental illness and sleep and circadian problem(s) were randomized to receive TranS-C plus usual care (TranS-C + UC) or usual care plus delayed TranS-C (UC-DT). Overall impairment, psychiatric symptoms, and sleep and circadian dysfunction were assessed at pre-treatment, post-treatment, and 6-month follow-up. RESULTS: Higher numbers of sleep and circadian problems, versus one problem, were associated with worse overall impairment, psychiatric symptoms, and sleep and circadian dysfunction (ps < 0.05, ω2 = 0.06-0.15). Diagnostic threshold was not associated with baseline functioning (ps > 0.05). TranS-C + UC versus UC-DT was associated with psychosocial and sleep and circadian improvements for specific sleep and circadian problems (insomnia, hypersomnia, parasomnias, periodic limb movement/restless leg syndrome, circadian rhythm disorders), though improvements varied by problem. TranS-C + UC outcomes were not moderated by number of sleep and circadian problems (ps > 0.05). CONCLUSION: Higher numbers of sleep and circadian problems, not diagnostic threshold, were associated with greater impairment. Transdiagnostic utility of TranS-C + UC was supported.