Recently atomic nitrogen densities of ~5¿107 to 108 cm-3 were inferred in the day-time thermosphere from studies of the NI(2D-4S) 5200¿ emission and from the photochemistry of various ion species using data measured by the Atmosphere Explorer-C satellite. In this paper we use the photochemistry of NO+ and O2+ at night to determine nocturnal N(4S) densities in the thermosphere. We present evidence for a missing source of NO+ and a missing sink for O2+ at night and show that this can be adequately supplied by the reaction O2++N→NO++O if the N density at 200 km is ~7¿106 cm-3. The atomic nitrogen has an important effect on studies of the 6300¿ airglow. The omission of N in calculations of O(1D) using ground-based data results in an overestimate of the rate coefficient of quenching of O(1D) by N2.