One hundred eighty nine hours of mesopause region temperature, meridional wind, and Na density measurements were made at the Urbana Atmospheric Observatory (40 ¿N, 88 ¿W) during both day and night in Sep/Oct 1996. The data are used to study tidal oscillations and their relationship to mesospheric temperature inversion layers. The observations show that thermospheric forcing by UV and EUV absorption plays an important role in establishing the tidal characteristics in the upper mesosphere. Because of the large tidal amplitudes and strong thermospheric heating during the day, the general characteristics of the background temperature structure can only be established by measuring temperature throughout the diurnal cycle. When the temperature profile is averaged over the complete diurnal cycle, the mesopause is considerably warmer (196 K compared to 190 K) and lower (96 km compared to 101 km) than that observed at night. Furthermore, there is little evidence of the prominent inversion layers frequently reported in nighttime measurements. These features appear to be artifacts associated with incomplete sampling of the strong diurnal tide. ¿ 1998 American Geophysical Union