An analysis of NFL injury data shows that players with concussions missed an average of 16 days last season, up from only four days in 2005.
The study by Edgeworth Economics, based on information collected by the league, shows that the average time out for other types of injuries has been more steady.
Severe injuries - causing a player to miss at least eight days - increased every season from 2009-12. Jesse David, the economist overseeing the study, says Wednesday "you now have more severe injuries overall" because of the hike in reported concussions.
He says longer absences for concussions could be because head injuries are more damaging or because players simply are being held out longer.
The NFL says it will look at the study.