The unnamed man had set himself up a makeshift bed in the trailer of his car and gone for a nap in it on Saturday afternoon at the Wacken music festival, daily newspaper the Süddeutsche Zeitung reported on Sunday.
He apparently did not notice that the trailer, which he had propped up with posts and covered with a tarpaulin, was parked close to a generator that was emitting carbon-monoxide, Der Spiegel magazine reported.
A spokeswoman from the nearby Itzehoe police in Schleswig-Holstein told the paper that the man had retired to his trailer to go to sleep at around 4pm. His friends found him around two hours later - dead.
Paramedics spent 30 minutes trying to revive him, but to no avail.
When the Süddeutsche Zeitung asked the Wacken organisers they said that they had no idea about the accident.
Wacken is one of the biggest heavy metal festivals in the world, and this year attracted over 75,000 visitors and 130 bands.