Fielder left camp Wednesday and traveled to Texas to take part in a sleep study after having trouble getting rest during spring training.
Sleep apnea is a potentially life-threatening condition that causes gaps in a person's breathing while sleeping, and Fielder told reporters that he stopped breathing 39 times per hour during his study. He said doctors told him that more than 30 is considered serious.
Fielder, who was in the lineup for Friday's game against Milwaukee, told reporters that he has been using a mask and a breathing device since taking part in the sleep study and has no immediate plans to have surgery.
"I wasn't panicking,'' Fielder said. "I just wanted to make sure I was OK. I felt like I was sleeping long, but I wasn't feeling rested. That was my concern, trying to figure out what was wrong with that. Now that I know what it is, it's good. I can breathe through the night now."
The star first baseman and designated hitter had been having problems sleeping since arriving at camp in Arizona.
"I think Prince had gone about his job where I had not noticed a whole lot, but obviously those type of things need to be communicated, which he did," Rangers manager Jeff Banister said Wednesday.
Fielder, who has one hit in nine at-bats this spring, was the 2015 American League Comeback Player of the Year, hitting .305 with 23 home runs, 28 doubles and 98 RBI. He missed most of the 2014 season, hitting three home runs in 42 games, and needed season-ending neck surgery.
The Associated Press contributed to this report.