The top-ranked American, who has not played since March, would awake from a deep sleep with his heart racing, a condition doctors described as a form of arrhythmia, the report in USA today said.
Fish, 30, pulled out of the French Open last week because of what organisers termed fatigue but, according to USA Today, he said he had corrective medical procedure performed last week.
"It felt like my heart was going to jump out of my chest," Fish, who has not played since March, told the USA Today in a phone interview from his Los Angeles home.
"During days, I'm totally fine. ... But every time I would go to bed my mind would start racing. Is this going to happen tonight? Is this going to be another night like that? It was super hard to go to sleep."
Fish, who said the first episode occurred prior to a Davis Cup win over Switzerland's Stanislas Wawrinka in mid-February, expects to resume light training this week with an eye on making a return at Wimbledon or the Queen's Club warm-up event.
"I want to stress that I'm good now, I'm fine, they've fixed the problem," Fish said.
"I don't want people to think I missed the French Open because I didn't want to go because I was just tired. That's just not the case."