Look up lights out in Wiktionary, the free dictionary.
In boarding schools and on trips or holidays that involve young people, the equivalent of bedtime is lights out or lights-out; a term also used in prisons, hospitals, in the military, and in sleep research.
^A Scottish prayer: "I am going now into the sleep, / Be it that I in health shall wake; / If death be to me in deathly sleep, / Be it that in thine own arm's keep, / O God of grace, to new life I wake; / O be it in thy dear arm's keep, / O God of grace, that I shall awake!" (from Poems of the Western Highlanders, 1900; in The Oxford Book of Prayer, general editor: George Appleton. Oxford University Press; no. 325 at p. 101