They had tested his blood-oxygen levels. They were low. Keller should have been experiencing all of these things, but he felt fine. One of the doctors recommended a sleep study to find an explanation.
It was then, back in 2012, that Keller was diagnosed with sleep apnea, a sleep disorder that affects 24% of adult men and 9% of women. Around the same time, Keller was taking part in CNN's Fit Nation Challenge
. He'd made a commitment to turn his health around, and that also included his sleep.
Keller certainly appreciates the value of sleep, maybe more than most. He's a long-haul truck driver, and his livelihood -- and life -- depend on his ability to stay awake, and alert.
"After finding out I had sleep apnea, of course I wanted to know a little more about it," Keller said. "And from what I understand, you're sleeping but you're not resting."
All Keller's life, he had no trouble sleeping through the night, but his sleep apnea was preventing him from a restful night's sleep. Obstructive sleep apnea occurs when tissue in the back of the throat collapses, blocking the airway. It is caused by excess weight, large tonsils, or even just the construction of someone's throat.
For anywhere from 10 seconds to a minute or more, the person will actually stop breathing, lowering blood-oxygen levels and straining the heart. This can happen hundreds of times throughout the night.
"Some people called it the snoring sickness," says Dr. William Dement. "You struggle to breathe ... and you wake up to breathe. If it's severe, you wake up all night long."
Dr. Dement has been nicknamed the father of sleep science. He began studying sleep in the 1950s when few people realized how much there was to discover, and quickly learned that sleep was complex -- filled with patterns and problems.
In the summer of 1970, Dement opened a sleep disorders clinic at Stanford University -- the first of its kind. From the start, the number one problem for his patients was obstructive sleep apnea.
With all those interruptions, Keller and others who suffer from sleep apnea never complete a full sleep cycle. Without reaching the last stage, known as rapid eye movement or REM, the body can't fully regenerate and rejuvenate. REM sleep was discovered by Dr. Dement, and it allows the body necessary recovery time needed for daytime function.
"When you're dealing with sleep apnea, although you're lying there all night and your eyes are closed, you never reach that restful stage of sleep," Keller said. "I was tired and I didn't realize I was tired, because that was all I've ever been -- was tired."
For Keller, that changed after his first sleep study. During the study, he was hooked up to a C-PAP machine. Any time he stopped breathing, the machine would force pressurized oxygen into his airways, forcing them open.
When he woke up the next morning, Keller said he "felt like a new man." For every night since, a C-PAP machine has been by his side, both at home and on the road.
"I know how important it is as a driver that I get good sleep care," Keller said. "After the level of sleep that I got, I welcomed [the C-PAP machine] with open arms!"
Keller spends more than 250 days on the road each year. He drives all hours of the day and night, and can see a difference in his overall health now that he's getting a full night's rest. He's also recently lost nearly 100 pounds, and his ultimate goal is to get off the C-PAP machine. If his sleep apnea is linked to his weight, Keller said, he will achieve that goal.
"I think it's kind of been taken for granted, sleep health," Keller said. "Sleep health is important enough that they want to consider it the third pillar of health that goes along with diet and exercise. The sleep kind of completes it."