Lidar observations of the mesospheric Na layer have revealed considerable diurnal variations particularly on the underside of the main layer where more than an order-of-magnitude increase of Na density has been observed below 80 km after sunrise. In this paper, multi-year Na lidar observations over a full diurnal cycle at Utah State University (USU) (41.8°N, 111.8°W) and a global atmospheric model of Na with 0.5 km vertical resolution in the mesosphere lower thermosphere (WACCM-Na) are utilized to explore the dramatic changes of Na density on the layer underside. Photolysis of the principal reservoir NaHCO<sub>3</sub> is shown to be primarily responsible for the increase in Na after sunrise, amplified by the increased rate of reaction of NaHCO<sub>3</sub> with atomic H, which is mainly produced from the photolysis of H<sub>2</sub>O and the reaction of OH with O<sub>3</sub>. This finding is further supported by Na lidar observation at USU during the solar eclipse (> 96 % totality) event on August 21<sup>st</sup>, 2017, when a decrease and recovery of the Na density on the underside of the layer were observed. Lastly, the model simulation shows that the Fe density underside around 80 km increases larger and earlier than observed Na changes during sunrise because of the considerably faster photolysis rate of its major reservoir FeOH.