A seat for a passenger in a vehicle, comprising a seat bottom and a seat back, for which a seat frame is supported by a base, by which the seat may be attached to the vehicle, said seat comprising at least one "seated" position, one "lying" position and at least one "resting" position, intermediate between the "seated" position and the "lying" position, in which: - a seat plate in the seat frame that can be moved in rotation around a rear axis of said seat, oriented in a transverse Y direction, horizontal to the seat and perpendicular to a vertical XZ plane of symmetry of the seat assembly, with said rear axis being positioned in an X longitudinal direction on the side of a rear edge of the seat; - the seat plate can be moved in rotation around a tilting axis which is fixed to the seat frame, parallel to the rear axis, positioned according to the X longitudinal direction, in front of said rear axis and between the rear edge of the seat and a front edge of the seat;- the seat frame is moved forward during a seat conversion from the "seated" position to the "lying" position, a conversion during which: + the tilting axis is moved on the base following a first descending trajectory in relation to the horizontal X longitudinal direction;+ the rear axis is moved on the base following a second continuous trajectory comprising successively the direction of movement toward the front of the seat; - an essentially horizontal rear segment; - an intermediate rear segment with a downslope greater than the slope followed by the tilting axis; - an intermediate forward segment with an upslope; - an essentially horizontal front segment; - a seat plate in the seat frame that can be moved in rotation around a rear axis of said seat, oriented in a transverse Y direction, horizontal to the seat and perpendicular to a vertical XZ plane of symmetry of the seat assembly, with said rear axis being positioned in an X longitudinal direction on the side of a rear edge of the seat; - the seat plate can be moved in rotation around a tilting axis which is fixed to the seat frame, parallel to the rear axis, positioned according to the X longitudinal direction, in front of said rear axis and between the rear edge of the seat and a front edge of the seat;- the seat frame is moved forward during a seat conversion from the "seated" position to the "lying" position, a conversion during which: + the tilting axis is moved on the base following a first descending trajectory in relation to the horizontal X longitudinal direction;+ the rear axis is moved on the base following a second continuous trajectory comprising successively the direction of movement toward the front of the seat; + the tilting axis is moved on the base following a first descending trajectory in relation to the horizontal X longitudinal direction;+ the rear axis is moved on the base following a second continuous trajectory comprising successively the direction of movement toward the front of the seat; - an essentially horizontal rear segment; - an intermediate rear segment with a downslope greater than the slope followed by the tilting axis; - an intermediate forward segment with an upslope; - an essentially horizontal front segment;