Doctors treating seven-times Formula One world champion Michael Schumacher will begin lowering his sedation level in order to start the process to wake him up, his agent said on Thursday.

Schumacher has been in an artificial coma in a hospital in the eastern French city of Grenoble for more than four weeks following an accident in late December when he slammed his head on a rock while skiing off-piste in the resort of Meribel.

"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.

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