The diurnal variation of the neutral gas heating efficiency due to photo-ionization processes &lgr;?1025 ¿ is calculated by using a diurnal model of the mid-latitude ionospheric F region. The absorbed solar energy is split between photoelectrons and ion pair production that becomes stored chemical energy. This energy is then transferred to the neutral gas through elastic and inelastic collisions of photoelectrons with the neutrals and through heating associated with exothermic ion-molecule chemical reactions. In addition, neutral gas heating also occurs by elastic and inelastic collisions between the electrons, ions, and neutrals due to thermal conduction of heat in the electron gas from the magnetosphere. Neutral heating by collisional processes between electron, ions, and neutrals is dominant above 300 km. Between 170 and 300 km, photoelectrons and chemical reactions are the major heat sources. Below 170 km the neutral heating is primarily due to the absorption of the Schumann-Runge continuum. The calculations also show that neutral gas heating due to absorbed airglow emissions is generally negligible for dayglow processes. The neutral gas heating efficiency due to EUV solar heating (&lgr;?1025 ¿) below 400 km lies between 30 and 35%. The calculated electron gas heating efficiency, below 250 km, where photoelectron heating predominates, is roughly 5%, and the ion heating efficiency is about 2% with respect to the absorbed EUV solar energy.