。
Night is when most people sleep.
Literal
Night [topic-は] most [gen-の] people [subj-が] sleep time [literary-である].
たいていの人 ('most people') — たいてい can be prenominal with の. 寝る時 ('the time of sleeping') is a relative clause modifying 時. The copula である gives this a literary/definitional feel, like an encyclopedia entry.