We present the first global measurement of the sodium mesospheric layer obtained from the processing of about 100 000 star occultations by the GOMOS instrument onboard the ENVISAT satellite. The retrieval method is developed on the basis of a simple DOAS retrieval applied to averaged transmittances. The vertical inversion of the sodium slant path optical thickness is performed by using a modified Gaussian extinction profile. A global climatology is derived by using monthly bins of 20 degrees in latitude. The high variability of the sodium layer is confirmed on a global scale as well as the presence of an important modulation in the annual cycle. Also, we present some evidence for the existence of a diurnal cycle characterized by an increase of the sodium concentration in daylight.