High-resolution temperature profile data collected at the Urbana Atmospheric Observatory (40ºN, 88ºW) and Starfire Optical Range, NM (35ºN, 106.5ºW) with a Na lidar are used to assess the stability of the mesopause region between 80 and 105 km. The mean diurnal and annual temperature profiles demonstrate that in the absence of gravity wave and tidal perturbations, the background atmosphere is statically stable throughout the day and year. Thin layers of instability can be generated only when the combined perturbations associated with tides and gravity waves induce large vertical shears in the horizontal wind and temperature profiles. There is a region of reduced stability below the mesopause between 80 and 90 km where the temperature lapse rate is large and the buoyancy parameter N2 is low. The vertical heat flux is maximum in this region which suggests that this is also a region of significant wave dissipation. There is also a region of enhanced stability above 95 km in the lower thermosphere where N2 is large. There appears to be little wave dissipation above 95 km because the temperature variance increases rapidly with increasing altitude in this region and the vertical heat flux is zero.