Now Jesus had spoken of his death: but they thought that he spake of taking rest in sleep. λέγει, though in the present tense, represents a time anterior to the time of ἔδοξαν. κοίμησις is found in Ecclus.
46:19. This is an explanation of the misunderstanding, occasioned, perhaps, by the statement of verse 4, and further elucidated by what follows. A difference prevails between κοίμησις and ὕπνος as both words are used for sleep; but the former has rather the idea of the repose accompanying sleep, the latter the phenomenon itself.
With one or two exceptions, κοιμᾶσθαι is always used in the New Testament of the sleep of death, ὑπνός never.