The deceased, who was not identified, had been snoring and stopped, said Jared Noall, a passenger and combat medic with the Army Reserve.
Noall said he didn't think anything of the man seated behind him not snoring.
"We just thought he had found the sweet spot where he was comfortable and could sleep," he told KSL-TV, Salt Lake City, Tuesday. That is, until Noall turned to look at the man and thought immediately something was wrong.
Noall nudged the man to see if he was still sleeping, then checked for a pulse. When he found none he began performing CPR with the help of a female passenger until the plane landed and paramedics arrived. The man, who appeared to be in his 30s, was pronounced dead after attempts to revive him for 30 minutes were unsuccessful.
"It sort of sucks because you always want to save people's lives, but what can you do?" Noall said. "Everyone came together to try to help, though. We did what we could."