Unfortunately, this is an issue which started since Mavericks, this is a thread on Apple forums:
https://discussions.apple.com/thread/5495375
It seems to be an issue with bluetooth < > wifi, and it seems to happen when both are enabled, and this is a theory: when you power off one of the bluetooth devices while the macbook is in sleep mode.
Disabling bluetooth fixes the problem, but this is not an option for most people (mouse, keyboard and so on).
I have this issue ever since Mavericks, and it's completely annoying
(Running late 2013 MBPr 15")