Proto-Indo-European Roots

Root/Stem: *nokw-, *nekw-
Meanings:  night, darkness
Cognates: Greek nuks (a night)
Latin nox (a night), genitive noctis
Common Celtic *nokti- (night) > 
Old Irish nochd, Welsh henoeth, Cornish neihur, Breton neyzor, nos, Irish anocht (tonight), Scottish nochd (tonight) 
Common Germanic *nahtiz (night) > 
Gothic nahts (night), Old Saxon neht, Old English niht, neaht, Old Norse natt, Old Frankish & Old Swedish nacht, Old High German naht
Hittite neku- (to get dark), nekuz (evening) - absense of suffix -t- shows the original IE root
Vedic nak- (night), Sanskrit nakti- (night), naktam (at night)
Albanian naté (night)
Old Baltic *nakti- (night) > 
Old Prussian naktin (acc. sg. night), Lithuanian naktis (night), Latvian nakts
Common Slavic *noktï 'night' > 
Belarussian noch, Bulgarian noshch, Serbo-Croatian and Slovenian noc', Chekh Slovak Polish and Sorbian noc, Polabian nu"c, Russian noch', Ukrainian nich
Notes: The feminine noun of i-stems, passed to consonant stems in some branches (e.g. in Germanic). 
The stem denoted everything connected with dark, getting dark, night and sunset. The suffix -t- used, obviously, for forming a noun. 
French has nuit (night) as Latin kt > it, Spanish has noches, and almost all Romance languages have similar forms. 
English preserved night < Middle English niht
German keeps the old form Nacht.