People who undergo screening colonoscopy in the morning rather than afternoon may be more likely to have potentially cancerous growths detected, a new study suggests.

Researchers at the Cleveland Clinic in Ohio found that among more than 3,600 colonoscopies performed at their center, the rate of polyp detection was higher with tests done in the morning.

The United States Preventive Services Task Force recommends colonoscopy every ten years starting at the age of 50, as a screening test for colon cancer. Polyps are benign growths, but because they have the potential to become cancerous, they are usually removed if they are caught during screening colonoscopy.

Polyps were found in 29 percent of patients who had a colonoscopy in the morning, versus 25 percent of those who were screened in the afternoon. There was also evidence that detection rates dipped as the day wore on, according to findings published the American Journal of Gastroenterology.

It is not clear why morning-time screenings had a higher detection rate in this study, according to the researchers, led by Dr. Madhusudhan R. Sanaka. They speculate, though, that doctor fatigue could be a factor.

In some cases, physicians performing the screenings "might have been less attentive or less vigilant in the afternoons compared with the mornings," the researchers write.

That, however, is just a theory, notes Dr. Joseph Vicari, in an editorial published with the study. "No data exist in this study to support that conclusion," writes Vicari, of Rockford Gastroenterology Associates in Illinois.

He also points out that the morning group of patients had a higher number of men, an older average age and a greater proportion of patients with a history of polyps than the afternoon group.

All three factors, Vicari writes, could have "skewed the data" toward a higher detection rate in the morning.

Sanaka's team agrees that no definitive conclusions can yet be drawn. "The reasons and implications of this finding should be studied further," the researchers write.

If physician fatigue is found to be a factor, Vicari notes, then changes in how colonoscopies are scheduled — fewer in the afternoon or fewer throughout the day, for example — might improve polyp detection rates.