The study of the CI in the nearby Universe and at high redshifts has shown that CI is a very powerful tracer of the molecular gas in the galaxy interstellar medium and starts giving an advantage at z>0.5 because it remains well excited for cooler lower density molecular gas and it is optically thin (no need of the XCO factor). When coupled with high-J CO emission lines the ratio, high-J CO/CI, is a good proxy of (warm, dense, star-forming (SF) gas)/(total H2 gas) mass fraction and provides a unique insight on the main source heating the gas and therefore indicating whether a system is merger driven or disc-like. I show observations of three lensed galaxies at z~3 obtained with the APEX/SEPIA5 receiver and a few ALMA observations towards high-z submm galaxies and derive conclusions on the dominant power source of the molecular ISM of these objects.