“Many conditions common to former NFL players such as sleep apnea, low testosterone, high blood pressure and chronic pain can cause problems with thinking, memory and concentration,” said lead investigator Rachel Grashow, a research scientist with the Harvard T. H. Chan School of Public Health.

A Harvard study of almost 2,000 former National Football League players revealed that about one-third believe they have chronic traumatic encephalopathy, a degenerative brain disorder linked to repeated head impacts.

The degenerative brain disease can only be diagnosed after death. But hundreds of retired players reported symptoms linked to CTE, like depression, mood swings and suicidal thoughts.

A new multi-year Harvard study found that 34% of former professional football players believe they have brain injuries from the sport. The respondents say they perceive themselves to have chronic traumatic encephalopathy ( CTE) according to the study published in JAMA Neuro on Monday.