Airglow originating from metastable helium He(2^3S) at 1083 nm has been used to study the upper thermosphere since its discovery in 1959, yielding insights into e.g. solar EUV intensities and thermospheric photoelectron densities. However, passive airglow measurements are fundamentally limited, because they represent a vertical integration of the entire He(2^3S) layer, spanning 100s of km, and because they rely on solar illumination of the He(2^3S) layer, so temporal changes are difficult to distinguish from vertical variations. Recently, the first height-resolved measurements of He(2^3S) density were made by fluorescence lidar, opening a new window for studying the upper thermosphere. We report on a series of 51 measurements by this instrument spanning an entire winter season and extending to an altitude of 1000 km, revealing a broad He(2^3S) layer that extends to, and peaks at, higher altitudes than previously expected.