Sleep mode will allow the unit to restart quicker. It needs to be in this mode for altitude acclimatization to work as the unit checks you elevation at midnight.
I believe that if WiFi is enabled that it will periodical check to see if anything is in the queue to be downloaded and will download it.
Don't know exactly, but I'd say it's not great. I've had it go dead between rides when I accidentally left it in "sleep" mode. I always try to make sure I do full power off instead. Then the battery lasts a very long time.
