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Introduction: The circadian timing system regulates diurnal sleep–wake rhythm. Previously, we showed that, in patients undergoing elective surgery, sleep–wake timing is altered and post‐operative sleep quality is reduced. However, how the timing of the surgical procedure affects the disturbance and what other factors affect this disturbance remain unknown. Methods: Single‐centre prospective observational study investigating the influence of surgery timing on post‐operative sleep in adult patients (≥18 years) undergoing elective surgery. Sleep–wake timing was measured from three nights before until seven nights after surgery with a daily sleeping log. Primary outcome was post‐operative midpoint of sleep shift between patients undergoing morning and afternoon surgeries. Secondary outcomes included factors affecting sleep timing disturbance and changes in subjective sleep quality. Results: We included 259 patients: 144 patients underwent morning procedures (08:00 h–12:00 h) and 115 underwent afternoon procedures (12:00 h–17:00 h). Both groups had significant phase advance of midpoint of sleep on the night after surgery when compared with three nights before surgery (mean − 00:41 h, 95% CI −00:27 h to −00:54 h, p <.001, for morning surgery and mean − 00:28 h, 95% CI −00:09 h to −00:46 h, p =.003, for afternoon surgery). However, there was no between‐group difference (mean − 00:13 h, 95% CI −00:35 h–00:09 h, p =.25). Reduction of sleep quality was also similar. Phase advance was larger for patients with an evening chronotype or with lower pre‐operative sleep quality. Decline of sleep quality after surgery was larger for patients with an evening chronotype, longer procedures or better pre‐operative sleep quality. Conclusion: Our results suggest that the timing of surgery between 08:00 h and 17:00 h does not modulate the effect of anaesthesia and surgery on phase of the sleep–wake rhythm in patients undergoing elective surgery with a low pre‐operative risk of delirium. Editorial Comment: Timing of surgery may impact post‐operative sleep. However, in this prospective cohort study of elective surgical patients, sleep–wake timing and post‐operative sleep quality did not differ between those undergoing morning versus afternoon surgery. [ABSTRACT FROM AUTHOR]