Sick children admitted to pediatric intensive care units (PICU) during the evening rather than during the day are slightly more likely to die in the first 48 hours of care, a study of 15 PICUs found.

Researchers did not determine why the difference exists, but fatigue, lighter staffing levels and less experienced doctors and nurses working night shifts could all be factors, said a senior author, UC Davis Children's Hospital pediatric critical care physician James P. Marcin. The study did not include UC Davis' PICU.

The study, which appeared appearing in Pediatrics, found that the mortality rate for children admitted during the day was 2.2 percent. But for children admitted during the evening — defined as 5 p.m. to 7 a.m. — the death rate increased to 2.8 percent.