The long-term dynamics of the Martian upper thermosphere near the exobase (~160-200 km) are still relatively poorly constrained by data. Electron reflectometry (ER) provides a way to derive, from electron loss cones, neutral mass densities at these altitudes in the night hemisphere. Because the Mars Global Surveyor Electron Reflectometer was not designed for this purpose, uncertainties in individual measurements are large and thus upper thermospheric variability can be characterized only on time scales of weeks or longer. Density measurements are presented at 2 A.M. local time and 185 km altitude, from April 1999 until November 2006, spanning ~4 Martian years. We observe a weaker correlation with lower atmospheric dust activity than is seen in the lower thermosphere and a weaker correlation with solar EUV flux than is observed in the dayside exosphere. Seasonally repeating features are (1) overall expansion/contraction of the nighttime thermosphere with heliocentric distance, (2) much lower densities at the aphelion winter pole compared to the perihelion winter pole, and (3) a short-lived local density maximum at aphelion in the southern hemisphere. Interannual differences are also observed; in particular, the interval of low densities in the southern winter occurs progressively later as solar EUV flux decreases from solar maximum to solar minimum. Results are compared with predictions from the Mars Thermosphere General Circulation Model and LMD Mars Global Circulation atmospheric model frameworks for Ls = 90°-180°, which generally underestimate and overestimate neutral densities, respectively. This disagreement reflects the difficulty in simulating nightside dynamical and cooling processes. Copyright 2010 by the American Geophysical Union.