A study is made of winter nighttime increases in the total electron content of the ionosphere over the North American region using data from several total electron content stations and from several geostationary satellites for the month of December for the years 1971, 1974, and 1975. The increases are preceded by substorm activity and enhancements in the total electron content of the ionosphere in the auroral regions. Total electron content nighttime increases are accompanied by large increases in the peak density of the F layer and a lowering of the layer height. At mid-latitudes most of the enhancements occur after midnight (local time) with a marked maximum around 0300 hours (local time). The spatial extent of the enhancements tends to increase with increasing levels of magnetic activity. The enhancements appear to occur at a constant local time, giving an apparent westward motion. The winter nighttime increases are not accompanied by large changes in the plasmaspheric content. Mechanisms to explain the winter nighttime enhancements are discussed. Low mid-latitude total electron content increases appear to be controlled by mechanisms different from those operating at higher latitudes.