We reanalyse the Ly-apha forest data from distant and bright quasar spectra at z\sim 5.0 that have already been considered in Viel et al 2013 and Irsic et al 2017. In particular, we study in details the degeneracy between the thermal history of the intergalactic medium and the warmness of warm dark matter in their effect on the flux power spectrum at the smallest scales. We consider a new suite of cosmological simulations with the most conservative thermal history compatible with current constraints on reionization of the IGM. We conclude that the observed cut-off is compatible with a dark matter constituted by sterile neutrinos of mass m=7keV. To put new robust constraints on the warmness of dark matter from this data the above mentioned degeneracy had to be removed. We consider possible ways to do it.