From kata and heudo (to sleep); to lie down to rest, i.e. (by implication) to fall asleep (literally or figuratively) -- (be a-)sleep.
see GREEK kata
1. to fall asleep, to drop off to sleep: Matthew 25:5.
2. to sleep;
a. properly: Matthew 8:24; Matthew 9:24 (on this and its parallels, cf. B. D. American edition, p. 1198{a}); b. euphemistically, to be dead: 1 Thessalonians 5:10; (Psalm 87:6 c. metaphorically, to yield to sloth and sin, and be indifferent to one's salvation: Ephesians 5:14; 1 Thessalonians 5:6.