Bruno has been training relentlessly ahead of a series of scheduled public appearances, including a candid television interview with Piers Morgan.
But concerns have developed for the 50-year-old, who has voluntarily gone to a clinic for exhaustion.
Bruno has battled with mental health issues before, and was sectioned under the Mental Health Act in 2003.
He split with his girlfriend recently, and close friends told the Sunday Mirror that he had been behaving erratically.
“He was running and doing two lots of four-hour gym sessions a day. Some nights he was getting up after just two hours sleep then going back to the gym,” a source told the paper.
“The Piers Morgan show is a big thing to do and he was worried that being on TV would add half a stone on to him. He wanted to look as good as possible.
“But lack of sleep and stress brings on bipolar in a big way and Frank and his family and friends were worried things were getting on top of him. He was showing signs of not being himself.”
Bruno reportedly told Morgan in the interview, due to be screened on Friday, of the devastation he felt after splitting with his wife in 2001.
“I was completely heartbroken," he said, "I had a nervous breakdown. It was the most horrible time.”
Bruno’s agent Dave Davies yesterday confirmed the ex-boxer was receiving treatment.
“Frank hasn’t been sectioned,” Davies clarified. “He started showing tendencies of bipolar. From what I understand he went of his own accord into a clinic.
“He needs a couple of weeks’ good rest. He is absolutely shattered. With his condition, Frank needs to take more care of himself.”