I've noticed this happening 5 times across 3 days. It's happening on a Haswell Retina MBP running Mavericks. Apple discussion forums are showing some people experiencing similar issues after Mavericks upgrade on other systems; iMacs and older MBPs included. I personally didn't experience this on my old MBP on Mavericks. There are no crash report upon re-login, but console logs show "System was rebooted due to sleep/wake failure". What I've done for now is to turn off Power Nap. This feature is new to Mavericks, and only supported on some newer systems, which may explain why my old MBP3,1 is not rebooting itself. Anyone else experiencing this?