David Powell, 50, was recovering from a lung operation when he lost his temper because Gordon Gullick, 72, was crying out in the night and disturbing his sleep.
Retired steelworker Mr Gullick, who was being treated for a chest illness, suffered serious head injuries.
He died from a blood clot three months later, but the coroner ruled his death was not linked to the attack.
Unemployed Powell of Brynithel, Blaenau Gwent, was convicted by a jury last month of wounding with intent to cause grievous bodily harm.
Cardiff Crown Court was told he and the victim were patients in Aberbeeg Hospital, near Abertillery, Gwent, in August 1997.
Mr Gullick had been moved into a single room next to Powell's ward before the attack because he was noisy.
![[ image: width=150]](/olmedia/190000/images/_193431_powell150.jpg)
The court was told Powell had taken a hospital spanner used for opening oxygen tanks and walked into the room.
He hit the grandfather four or five times, saying later he wanted to "quieten him down".
After his arrest Powell claimed he had not intended to cause him injury.
"I had had enough", he said. "I hit him four or five times until he quietened down. Then I stopped. After all, I didn't want to kill him. I just wanted him to be quiet."
Sentencing him, Judge Peter Jacobs said Powell had been in trouble for acts of violence before.
"I have come to the conclusion that a substantial prison sentence has to be served in this case," he said.
The judge told Powell he would actually serve 21 months because of his attempts to rehabilitate himself.
Afterwards his brother Michael Powell said he regretted the attack.
"He's emotional about it of course and he's very sorry about what he's done. He knows he did wrong," he said outside the court.
Mr Gullick's family are considering legal action against the hospital.
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