We know, the size of H atom is much smaller than that of Na atom. Due to which electrons are situated at a distance in Na from the nucleus as compared to that of H atom. As a result nuclear attraction received by the valence electrons is much higher for H atom than Na atom. So, there would be difficulty in removing a valence electron from H atom than Na atom due to higher nuclear attraction, so more energy would be required. Thus, ionisation enthalpy of hydrogen is higher than sodium.