Former Manchester City defender is in hospital after suffering heart trouble following a training session with Valenciennes.
The centre-back, who also played for Sheffield United, was on a bench taking in fluid and talking to team-mates when he fell backwards and lost consciousness.
A club doctor used a defibrillator on Sommeil, with L'Equipe reporting the player was given mouth to mouth by worried team-mates.
The 34-year-old was then taken to the city's hospital, where he is being looked after.
A statement released by the club read: 'According to medical staff at VAFC, David Sommeil has been the victim of a heart problem bringing about a loss of consciousness.
'He has been admitted to the intensive care unit of the Centre Hospitalier de Valenciennes.'