Jesus had been speaking of his death, but his disciples thought he meant natural sleep.
Remember that John, one of the disciples of Christ is writing this account and admitting that he and the others did not understand what Jesus was saying. Jesus teaches us bit by bit and will never give us more than we can absorb at any one time. His revelation to us is an on-going thing. Every time we go to Him with an open Bible and an open mind, He will show us something we have not seen before.