Using the Global-scale Observations of the Limb and Disk (GOLD) and the Global Ultraviolet Imager (GUVI), we examine the impact of sudden stratospheric warmings (SSWs) on the changes of thermospheric composition during the 2018-2019 and 2020-2021 Arctic SSWs and the 2019 Antarctic SSW. Contributions of planetary waves, gravity waves, and migrating tides are assessed by performing numerical experiments with the NSF National Center for Atmospheric Research (NCAR) vertically extended version of the Whole Atmosphere Community Climate Model (WACCM-X). The wind evolution simulated in WACCM-X aligns well with the quasi-geostrophic wind derived from Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) geopotential height measurements during Arctic and Antarctic SSWs. The variations in column integrated O and N2 density ratio (∑𝑂/N2) are generally similar among WACCM-X, GOLD, and GUVI observations. Following the onset of SSWs, ∑𝑂/N2 is reduced by ~10% over 50°S-50°N and enhanced by ~20% at higher latitudes. The ∑𝑂/N2 changes are associated with the reversals of the mean meridional circulation in the lower thermosphere, mainly driven by westward-travelling planetary waves. The results highlight that planetary wave activity during SSWs can significantly impact the mean state of the thermosphere.