Old English niht (West Saxon neaht, Anglian næht, neht) "night, darkness;" the vowel indicating that the modern word derives from oblique cases (genitive nihte, dative niht), 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 nuks "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 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]
Compare German Weihnachten "Christmas." 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. The Greeks, by contrast, counted their days by mornings.
To work nights preserves the Old English genitive of time. Night shift is attested from 1710 in the sense of "garment worn by a woman at night" (see shift (n.1)); meaning "gang of workers employed after dark" is from 1839. Night soil "excrement" (1770) is so called because it was removed (from cesspools, etc.) after dark. Night train attested from 1838. Night life "habitual nocturnal carousing" attested from 1852.