NIGHT, that part of the natural day of twenty-four hours during which the Sun is below the horizon, the dark part of the day from sunset to sunrise (see DAY). The word in O. Eng. takes two forms, neaht and night, the latter form apparently being established by about the 10th century. The word is common in varying forms to Indo-European languages. The root is usually taken to be nak-, to perish, the word meaning the time when the light fails (cf. Gr. yeww, Lat. nex, death, nocere, to hurt). It was customary to reckon periods of time by nights, and we still use " fortnight " (O. Eng. feouierlyne niht, fourteen nights), but " se'n-night " (seven nights) has been displaced by " week " (q.v.).
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)