Rangers slugger Prince Fielder hasn’t been sleeping too well lately. The Texas star left the team Wednesday to undergo sleep testing. After 48 hours of testing, results revealed the reason for Fielder’s problems: He suffers from sleep apnea.

The six-time all-star complained about having difficulty with getting rest and exams revealed why. During the study, Fielder stopped breathing 39 times per hour while asleep.

“I wasn’t breathing in my sleep,” Fielder told reporters Friday. “They gave me a mask so I could sleep. They said if you stop breathing 30 times an hour while you’re sleeping, that’s extreme. I was at 39.”

The mask treats the disorder by providing “a positive flow of air into the nasal passages in order to keep the airway open,” according to the National Sleep Foundation.

“Because I’m not breathing, I wake myself up all the time as if I was having a nightmare,” Fielder said. “But it’s just me not breathing, and they told me when that happens, your body uses adrenaline to wake up because you’re not breathing and it’s panicking. Then, I can’t go back to sleep and when I do, it’s just constantly like someone covering your mouth while you’re sleeping.”

Sleep apnea was linked to the untimely deaths of Pro Football Hall of Famer Reggie White and former Washington Redskins linebacker Kevin Mitchell. After trying a mask, mouthpiece and medication, Cleveland Indians first baseman Mike Napoli underwent surgery to treat his sleep apnea following the 2014 season.

“My wife told me I always snored loudly and stopped breathing in my sleep,” Fielder said. “You don’t feel it as much when you’re young, but as you get older, it affects you more. I wasn’t panicking, but I wanted to make sure.”

In 2015, Fielder hit .305 with 23 home runs and 98 RBI, a down year by his standards. “Believe it,” Fielder said when discussing if better sleep will lead to increased productivity on the diamond.

Risk factors of sleep apnea include being male, overweight, a family history of the disorder and a narrowed airway, according to the Mayo Clinic.