PARIS, Jan 30 (Reuters) - Doctors treating seven-times Formula One world champion Michael Schumacher for injuries suffered during a skiing accident have started lowering his sedation level to wake him up from an artificial coma, his agent said on Thursday.
Schumacher slammed his head on a rock while skiing off-piste in the French Alps resort of Meribel on Dec. 29.
The retired racing driver, the most successful Formula One champion of all time, has been in a stable but critical condition for more than four weeks in a hospital in the eastern French city of Grenoble.
"Michael's sedation is being reduced in order to allow the start of the waking-up process, which may take (a) long time," his agent, Sabine Kehm, said in a statement.
The accident prompted an outpouring of concern from fans of the German racing driver, who has undergone two operations.
A spokeswoman for the hospital declined to make any further comment on Schumacher's condition on Thursday.
Investigators have been trying to determine how fast Schumacher was skiing when he hit the rock. French police have said the slope was properly marked.
Schumacher, who turned 45 earlier this month, won a record 91 Grand Prix victories.
He left the sport last year after a disappointing three-year comeback with Mercedes following an earlier retirement from Ferrari at the end of 2006. (Additional reporting by Julien Pretot; Reporting by Alexandria Sage; Editing by James Regan and Andrew Heavens)