Rise And Shine!

In John’s Gospel we hear that Jesus told the disciples, “Our friend Lazarus has fallen asleep; but I am going there to wake him up.” John then tells us, “Jesus had been speaking of his death, but His disciples thought He meant natural sleep. So then He told them plainly, ‘Lazarus is dead.’” So why would Jesus use this picture of Lazarus 'sleeping' when in fact He knew that His friend had died?