Background: We sought to assess the influences of sleep duration, sleep adequacy, and daytime sleepiness on survival outcomes among Stage III colon cancer patients. Methods: We conducted a prospective observational study of 1175 Stage III colon cancer patients enrolled in the CALGB/SWOG 80702 randomised adjuvant chemotherapy trial who completed a self-reported questionnaire on dietary and lifestyle habits 14–16 months post-randomisation. The primary endpoint was disease-free survival (DFS), and secondary was overall survival (OS). Multivariate analyses were adjusted for baseline sociodemographic, clinical, dietary and lifestyle factors. Results: Patients sleeping ≥9 h—relative to 7 h—experienced a worse hazard ratio (HR) of 1.62 (95% confidence interval (CI), 1.01–2.58) for DFS. In addition, those sleeping the least (≤5 h) or the most (≥ 9 h) experienced worse HRs for OS of 2.14 (95% CI, 1.14–4.03) and 2.34 (95% CI, 1.26–4.33), respectively. Self-reported sleep adequacy and daytime sleepiness showed no significant correlations with outcomes. Conclusions: Among resected Stage III colon cancer patients who received uniform treatment and follow-up within a nationwide randomised clinical trial, very long and very short sleep durations were significantly associated with increased mortality. Interventions targeting optimising sleep health among indicated colon cancer patients may be an important method by which more comprehensive care can be delivered. Trial registration: ClinicalTrials.gov Identifier: NCT01150045.