Lidar observations obtained throughout the diurnal and annual cycles at Urbana, Illinois (40 ¿N, 88 ¿W), are used to examine the structure of mesospheric Na between 76 and 108 km. The Na layer is strongly influenced by seasonal and diurnal variations in solar illumination and by seasonal variations of mesospheric temperatures. Although there is little evidence of direct tidal perturbations in Na density, 24 hour oscillations dominate the diurnal variations. Na abundance and densities above 90 km are controlled by photoionization and charge exchange reactions during the day and recombination at night. The abundance is maximum at sunrise, and the peak-to-peak diurnal variation averages more than 30%. Below 85 km, density variations are controlled by photochemistry which interrupts the conversion of Na to the bicarbonate reservoir, and by warmer temperatures which enhance the liberation of Na from the reservoir. In this region the peak-to-peak diurnal Na variations can exceed 200%. The combined effects of photoionization above 90 km and photochemistry below 85 km induce a strong 24 hour oscillation in the layer centroid height. The peak-to-peak variation exceeds 1 km. The lowest centroid height is at local noon when Na densities below 85 km are maximum and photoionization above 90 km is strongest. The seasonal variations in Na densities and abundance are influenced primarily by changes in mesopause region temperatures which are coldest during midsummer. Below 95 km, reactions leading to the bicarbonate sink and the liberation of Na from this reservoir dominate Na chemistry. These reactions and their temperature dependencies are responsible for the large annual variation in Na abundance (2.3¿109 to 5.3¿109 cm-2). The measured correlation between Na abundance and the layer weighted temperature is +0.89. The ratio of the abundance to temperature variations is 1.9¿108 cm-2 K-1. The seasonal Na density and temperature perturbations are well-correlated (+0.6 to +0.9) below 98 km and negatively correlated above (-0.2 to -0.5). ¿ 1999 American Geophysical Union