They thought that Jesus was talking about actual sleep, but He was speaking about the death of Lazarus.
Berean Standard Bible (BSB)
Now Jesus had spoken of his death, but they thought that he spoke of taking rest in sleep.
World English Bible (WEB)
Howbeit{G1161} Jesus{G2424} spake{G2046}{(G5715)} of{G4012} his{G846} death{G2288}: but{G1161} they{G1565} thought{G1380}{(G5656)} that{G3754} he had spoken{G3004}{(G5719)} of{G4012} taking of rest{G2838} in sleep{G5258}.
Strong's King James
(13) But Yeshua spoke of his death but they thought that He spoke about the slumber of sleep.
The F.O.G
The F.O.G Message
but Jesus had spoken about his death, but they thought that about the repose of sleep he speaketh.
Young's Literal Translation