Purpose <p>Both short (&lt; 7&#xa0;h) and long (&gt; 9&#xa0;h) sleep duration are associated with poor health. Yet, little is known about sleep duration patterns and correlates in young adult childhood cancer survivors (CCS).</p> Methods <p>Data were from Project Forward, a population-based survey of young adult CCS in Los Angeles County. Average sleep duration, demographic and medical characteristics (e.g., treatment intensity), and mental (i.e., well-being, depressive symptoms) and physical (i.e., general health, comorbidities) health were measured via self-report. Chi-square and one-way ANOVAs assessed correlations between characteristics and short and long sleep duration (versus recommended; 7–9&#xa0;h). Multivariable linear and negative binomial regressions examined relationships between sleep duration and mental and physical health.</p> Results <p>Participants (<i>N</i> = 1,062, Mean age = 26.2&#xa0;years, 50% female, 51% Hispanic) were, on average, 14.5&#xa0;years (range: 5–22) post-diagnosis. Overall, 30% reported short and 7% reported long sleep. Short sleep was associated with older age, being employed, and having children (<i>p</i>s ≤ .001). Unemployment and lower educational attainment were associated with long sleep (<i>p</i>s ≤ .001). Both short and long sleep were associated with lower well-being (<i>p</i>s ≤ .001), greater depressive symptoms (<i>p</i>s ≤ .001), poorer self-rated general health (<i>ps</i> ≤ .012), and more comorbid conditions (<i>ps</i> = .001), compared to recommended sleep duration. Associations with physical health remained largely significant controlling for depressive symptoms (<i>p</i>s &lt; .025).</p> Conclusions <p>We found unique correlates of short and long sleep duration and associations with poorer mental and physical health among a large, diverse sample of young adult CCS.</p> Implications for Cancer Survivors <p>As a modifiable behavior, sleep duration is a relevant intervention target for improving outcomes after cancer.</p>