Seat convertible into bed having a deformable armrest

The seat (1), made with a back (2) that reclines to form a couchette, has a folding arm rest arrangement with a pivoted upright (4) and horizontal member (5) that lie flat and in line with one another when the seat is fully reclined. The arm rest is folded and deployed automatically by a lever (8) connected to the seat back and a pivot (9) on its base, and a power cylinder (27) between the lever and a sliding bracket (34, 36) under the arm rest horizontal member.