The nighttime thermal structure of the mesosphere and lower thermosphere (MLT) region at the Starfire Optical Range (SOR), New Mexico (35.0¿N, 106.5¿W), and at Maui, Hawaii (20.7¿N, 156.3¿W), are characterized using Na lidar observations. Both locations exhibit mesospheric temperature inversion layers (MILs) between 85 and 100 km that are not predicted by the MSIS-00 model. The amplitudes of the Maui MILs (~5.8 K) are about half of those at SOR (~9.8 K), and the Maui MILs have a smaller width (~11.1 km) compared to the SOR MILs (~14.5 km). The Maui lidar temperatures are generally warmer than the MSIS-00 predictions, while the SOR lidar data are comparable to the MSIS-00, except in the MIL altitude range. Both SOR and Maui mesopause temperatures are coldest in midsummer and are warmest during the mesopause transition periods. However, the Maui mesopause is warmer than the SOR, and the amplitude of the mesopause temperature variations at Maui (~9 K) is much smaller than at SOR (~19 K). Two distinct levels of mesopause altitudes are clearly shown in the SOR seasonal data with a low altitude around 86.5 km in summer (May through August) and a high altitude around 101 km during the rest of the year. Abrupt transitions between the two stable levels occur in early May and early September. The lidar measurements indicate a low mesopause altitude near 87.5 km in July at Maui when averaging over a 10-hour period centered at local midnight.