Colonoscopy is the gold standard for colorectal cancer prevention, yet small, flat, and sessile serrated polyps are frequently missed. Operator vigilance is crucial, and fatigue during prolonged sessions may reduce detection, particularly of subtle lesions. This study evaluated whether colonoscopy timing during the day affects polyp detection.

We conducted a prospective study of 47 consecutive patients undergoing screening or surveillance colonoscopy with adequate bowel preparation (Boston score ≥6) between January 2025 and September 2025. Procedures were classified as early session (first half of the program) or late session (after ≥3 consecutive exams). For each patient, polyps were recorded for number, size, morphology (Paris classification), location, and histology. Comparative analysis assessed detection differences between early and late sessions.

25 early and 22 late session colonoscopies were analyzed. Early session colonoscopies detected 65 polyps (mean 2.6 per patient), including 33 adenomas, 20 sessile serrated lesions, and 12 hyperplastic polyps. Late session colonoscopies detected 28 polyps (mean 1.3 per patient), mainly small, flat, with 60% in the right colon. Detection of small, flat, and sessile serrated lesions decreased by ~50% in late sessions. Procedure times and adverse events were similar. The reduction in detection during late sessions was statistically significant (p = 0.02).

Colonoscopy timing significantly affects lesion detection, with late-session procedures yielding fewer polyps, particularly small, flat, and sessile serrated lesions in the right and transverse colon. Awareness of endoscopist fatigue is essential for session planning, training, and quality assurance. Limiting consecutive procedures, scheduling breaks, and rotating operators may improve detection and patient safety. Larger multicenter studies are warranted to validate these findings and guide practical recommendations.