Another motorist pulled up alongside the black Audi A6, on Alfreton Road, at 8.40pm on May 7, and saw Adrian Miller asleep through the open window.
Prosecutor Sharioz Ahmed said the engine was still running and the handbrake was not engaged.
"It looked like he was unconscious,” the motorist said. “I tried to wake him up by shouting and beeping my horn - but he didn't respond.
"He was completely unconscious. I shook him but he just mumbled."
Miller was arrested at the scene and a breath test later revealed he had 91 mcgs of alcohol when the legal limit is 35 mcgs.
Treve Lander, mitigating, said Miller, a father-of-three and a warehouse supervisor, has never been in trouble with the police before.
"He tells me he has had drink problems in the past but there's nothing to suggest it's got a grip on him now,” he said. "91 isn't a reading you would associate with someone who has a drink problem."
Mr Lander told the court it was possible Miller, who is very close to being diabetic, could have suffered a hyperglycemic attack after eating a McDonald's meal and drinking alcohol.
“It’s out of character,” Mr Lander added. “He's not had memory loss before.”
Miller, 38, of Hilltop Road, Pinxton, admitted drink driving when he appeared at Mansfield Magistrates Court, on Wednesday.
Sentencing was adjourned for a probation report.