San Francisco 49ers running back Jarryd Hayne has opened up about the concerns of head injuries in the NFL, describing the heavy hits as "scary".
Hayne swapped Australia's Rugby League for life in the NFL ahead of the 2015 season and the former Parramatta Eels star admitted he has been taken aback by the impact of head injuries in the sport.
Concussion and brain trauma is a heated topic in the NFL, following the deaths of a number of former players due to football-rated degenerative brain diseases.
And Hayne, who said he would consider a NRL comeback with the Eels should he ever return to his home nation, revealed he struggled to sleep after games as a result of the heavy hits in his first season in American football.
"When I came off the full game I played in Detroit, my head was ringing," the 27-year-old said in an interview with Triple M's Grill Team in Sydney.
"That was obviously my first time being out there for that long. Literally, it was hard to sleep.
"Your head, your neck, your shoulders, people don't realise that just because you've got a helmet on, doesn't mean it helps your brain or anything like that.
"Your brain is still hitting your skull. The couple of head on hits that I had, they knock you about.
"The NRL is pretty endurance based, so you feel exhausted, but then the NFL, them hits, the impact that they have."
Hayne added: "I wouldn't say I'm worried, but there is definitely that kind of impact.
"As an athlete you kind of put your body in those situations because you're prepared to do whatever it takes.
"Everyone does that. It is scary. For me, after that full game I played, I was like, 'I don't know how a running back can do that for 10 years.' It's some heavy hits."