After the plane landed a ramp agent came out from the front cargo hold, which Alaska says is pressurized and temperature-controlled.
"Upon exiting, he told authorities he had fallen asleep," the airline said.
The worker, an employee of Menzies Aviation, walked off the plane and appeared OK, but was taken to a hospital as a precaution.
Passenger Marty Collins told KOMO-TV in Seattle the flight was surrounded by emergency vehicles when it returned to the airport.
She said passengers did not hear the worker banging or making noise.
"Nobody on the plane heard anything like that, nobody knew why we were turning around," Collins told the station. "They just said we were fine and we weren't in any danger."
Passengers were later told about the situation.
"They just said there was someone in the cargo hold and he's been escorted off and taken away," she said.
The flight departed again at 3:52 p.m., according to the airline's website.
