A breathing device, comprising a mouthpiece forming a breathing channel, to form a connection between a first end and a second end of the mouthpiece; the first end being configured for a user breathing into the mouthpiece through a breathing opening; an at least partly flexible rebreathing air chamber attached to the second end of the mouthpiece, thereby being in fluid connection with the breathing channel; the rebreathing air chamber being formed by at least partly flexible wall section(s), the at least partly flexible rebreathing chamber having at a first wall section, being permeable to gas by a plurality of pores provided in said wall section and/or the mouth piece comprising one or more though going openings.