I have an irritating problem with my 2013 MBA 13 (i7/8GB). I've found that sometimes when I sleep by closing the screen (as opposed to via the Apple menu or power button), that it doesn't wake up again. I can only start the machine by powering down and restarting. This only happens occasionally, but when it does it often leads to data loss of unsaved work. I've had a look at the system logs, but can't see anything that looks obviously wrong, but there is an awful lot of system messages in there! I'm going to try to keep track of exactly what is running before each time I close the screen, and see whether the same problem occurs is I sleep via the menu or power button. Has anyone else seen this behaviour?