(CNN) — It's estimated 20 to 30 percent of patients complain to their doctors about feeling fatigued. It could be a sign of a more serious problem.

Three decades of driving a taxi at all hours of the day and night left Stephen Belcher feeling exhausted.

“I just thought I was just tired and maybe if I get a good night's sleep on the weekend I'll feel better,” he said.

That didn't help.

It turned out that Stephen was more than just tired; he was suffering from fatigue.

"Most of the times you know why you're tired and the tiredness goes away with rest and sleep," explained Dr. Susmita Parashar with the Emory School of Medicine. "Fatigue on the other hand is a persistent, daily lack of energy impairing your ability to do normal activities."

Dr. Parashar calls fatigue an important marker that shouldn't be ignored.

It's a symptom in many conditions including: anemia, under-active thyroid, depression, heart disease and cancer.

In Stephen's case, he was diagnosed with a myriad of ailments from sleep apnea to emphysema to a heart blockage.

"If he hadn't come to the hospital it would have been a fatal condition so he really saved his life," Dr. Parashar said.

With proper treatment and medication for his ailments, Stephen reports he's feeling better.

And he's back in the swim of things now that he's not so tired.