Keep spawning in world at 256 blocks high

Sometimes when I load a world I have played in before, I am spawned at the world spawn, but at build height.  I was playing in survival both times this happened, so I "fell from a high place."  Also, since I had slept in a bed before this happened, I had to travel all the way back to the spawn point to retrieve my stuff.  It would be very inconvenient for this to keep happening.  Could somebody try to fix it?