nocturnal adj.

late Old English niht (West Saxon neaht, Anglian næht, neht) "the dark part of a day; the night as a unit of time; darkness," also "absence of spiritual illumination, moral darkness, ignorance," from Proto-Germanic *nahts (source also of Old Saxon and Old High German naht, Old Frisian and Dutch nacht, German Nacht, Old Norse natt, Gothic nahts).

The Germanic words are from PIE *nekwt- "night" (source also of Greek nyx "a night," Latin nox, Old Irish nochd, Sanskrit naktam "at night," Lithuanian naktis "night," Old Church Slavonic nosti, Russian noch', Welsh henoid "tonight"), according to Watkins, probably from a verbal root *neg- "to be dark, be night." For spelling with -gh- see fight.  The vowel indicates that the modern English word derives from oblique cases (genitive nihte, dative niht).

The fact that the Aryans have a common name for night, but not for day (q.v.), is due to the fact that they reckoned by nights. [Weekley]

Thus in Old English combinations night was "the night before (a certain day or feast day);" compare German Weihnachten "Christmas," literally "holy night." In early times, the day was held to begin at sunset, so Old English monanniht "Monday night" was the night before Monday, or what we would call Sunday night; so saeterniht "Friday night." The Greeks, by contrast, counted their days by mornings.

To work nights preserves the Old English genitive of time. Night soil "excrement" (1770) is so called because it was removed (from cesspools, etc.) after dark. Night train is attested from 1838; night-school from 1520s; night-life "habitual nocturnal carousing" is attested from 1852.

also nocturne, name of a division of the office of matins said just before daybreak (in the early Church a service recited after midnight), c. 1200, from Old French nocturne "evening service; curfew," from Medieval Latin nocturna, "group of Psalms used in the nocturns," from Latin nocturnus "pertaining or belonging to the night" (see nocturnal).

1851, "musical composition of a dreamy character," properly instrumental, from French nocturne, literally "composition appropriate to the evening or night," noun use of Old French nocturne "nocturnal," from Latin nocturnus (see nocturnal). The style and the name are said to have originated c. 1814 with Irish-born composer John Field, who wrote many of them, in a style that Chopin mastered in his own works, which popularized the term. But Field's work seems to have been appreciated in German and French publications before it came to attention in England in 1851.