First find your current sleep setting by opening Terminal in OS X and entering this at the prompt:
pmset -g | grep hibernatemode
That should return you something like “hibernatemode 3”. Remember this number, send an email to yourself, write it down on a scratch pad, whatever it takes to remember your default mode. Mode 3 keeps your RAM powered during sleep to allow super fast wake-up, but also writes an image file of all memory onto disk in case power is lost.
To change the hibernate safe sleep setting to not create an image file on the disk, i.e. mode 0 (mode zero, not the letter ‘o’), enter the following in a Terminal window:
sudo pmset -a hibernatemode 0
Enter your password when asked to do so. This prevents Safe Sleep from saving your memory contents to disk, in large part the cause of not being able to wake MacBook’s from sleep.
If you’d like to get back about a gigabyte or more of disk space, delete the memory image file with the following Terminal command:
sudo rm /var/vm/sleepimage
Macworld has a great article with more information about safe sleep and hibernation on MacBooks.