Sodium lidar measurements obtained between December 1980 and May 1986 at Urbana, Illinois, are compared with other reported lidar measurements and with recently developed models of the layer. Sodium abundance at Urbana reaches a maximum value in November, December and January which is approximately 4.5 times larger than the June minimum value of 2.15¿109 cm-2. Also in November and December, the layer centroid height is about 1.5 km lower than the yearly average value of 92 km. There is no significant seasonal variation of the rms layer width which has an average value of approximately 4.25 km. The peak-to-peak variations of the centroid height and rms width of the average nocturnal layer are 1 km and 600m, respectively. The semidiurnal tide is primarily responsible for a 36% peak-to-peak abundance variation in the average nocturnal layer which is compatible with a 20 cm s-1 amplitude for the vertical wind velocity. The vertical phase velocity of the density perturbations is approximately 1 m s-1 which implies a 45 km vertical wavelength for the semidiurnal tide. Because of gravity waves and tides the structure of the nocturnal layer changes substantially throughout the night. Variations of over 200% in abundance, 2 km in centroid height, and 1 km in rms width have been observed within the time span of a few hours.