We propose a nighttime source of metastable oxygen atoms O(1D) in the mesosphere. Here O(1D) atoms are generated by electron impact dissociation of O2 molecules in sprite halo events, where electrons are accelerated in strong electric fields associated with lightning discharges. The peak production rate of O(1D) in a single sprite halo event is estimated to be as large as 104 cm-3 for a duration of 1 ms, which is comparable to, or even larger than, the steady rate by solar-UV radiation. We also discuss the altitude dependence of O(1D) production in sprite halos and the dependence on day and night.