why does my MacBookPro return to a choppy looking home screen when falling asleep?

If I leave my computer to perform a task or just let it go to sleep but it often crashes and little square bits show glimpses of the home log in screen. If I run my mouse over the bits, more of the log in screen shows up and I can log in again.

Is there a setting in Energy Preferences or somewhere to stop this or is a glitch?