The Na layer in the upper atmosphere is very sensitive to solar radiation and varies considerably during sunrise and sunset. In this paper, we use the lidar observations and an advanced model to investigate this process. We found that the variation is mostly due to the changes in several photochemical reactions involving Na compounds, especially NaHCO₃. We also reveal that the Fe layer in the same region changes more quickly than the Na layer due to a faster reaction rate of FeOH to sunlight.