Etymology
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sight (v.)
1550s, "look at, view, inspect," from sight (n.). From c. 1600 as "get sight of," 1842 as "take aim along the sight of a firearm." Related: Sighted; sighting.
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sight (n.)

Middle English sight, from Old English sihð, gesiht, gesihð "thing seen; power or faculty of sight; aspect; vision; apparition," from Proto-Germanic *sekh(w)- (source also of Danish sigte, Swedish sigt, Middle Dutch sicht, Dutch zicht, Old High German siht, German Sicht, Gesicht), stem that also yielded Old English seon (see see (v.)), with noun suffix -th (2), later -t (14c.).

Verily, truth is sight. Therefore if two people should come disputing, saying, 'I have seen,' 'I have heard,' we should trust the one who says 'I have seen.' [Brhadaranyaka Upanishad 5.14.4]

Meaning "perception or apprehension by means of the eyes" is from early 13c. Meaning "device on a firearm to assist in aiming" is from 1580s. A "show" of something, hence, colloquially, "a great many; a lot" (late 14c.). Sight for sore eyes "welcome visitor" is attested from 1738; sight unseen "without previous inspection" is from 1892. Sight gag first attested 1944. Middle English had sighty (late 14c.) "visible, conspicuous; bright, shining; attractive, handsome;" c. 1400 as "keen-sighted;" mid-15c. as "discerning" (compare German sichtig "visible").

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night-night 
nursery talk, "good-night," 1896; form nighty-night is attested from 1876.
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night (n.)

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.

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night-watch (n.)
"guard kept during the night," late Old English; see night + watch (n.).
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night-light (n.)
1640s, "faint light visible in the sky at night," from night + light (n.). As "small light used in rooms at night to keep them from total darkness" from 1851.
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night-hawk (n.)

from 1610s in reference to various birds, especially the nightjar, from night + hawk (n.). Figurative sense of "one who stays up and is active at night" is from 1818.

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night-time (n.)

also nighttime, "the hours of darkness," late 13c., from night + time (n.). In the same sense Middle English had also nighter-tale (c. 1300), probably based on Old Norse nattar-þel.

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good-night 
phrase in parting for the evening or retiring to sleep, c. 1200, from good (adj.) + night. As an exclamation of surprise from 1893.
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night-shift (n.)

1710, "garment worn by a woman at night," from night (n. ) + shift (n.2). The meaning "gang of workers employed after dark" is attested from 1839, from shift (n.1).  

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