Well, when the lid is shut, it should go to sleep, but how can you be sure if there is no blinking indicator like those on MBP? Besides, this thing runs totally silent with SSD, so as long as the fan doesn't turn on, you won't be able to find out for sure. I'm asking because wife's school requires Windows, so I put Win 7 on boot camp, and she only uses win 7. One day I looked at the event viewer and noticed the machine actually woke up many times when the lid was shut. Eventually I figured out it was due to the unintended movement or clicks by a paired bluetooth mouse. After turned that feature off, it seems it hasn't woken up again with lid shut. I'm not concerned about battery, but definitely don't feel comfortable this thing wakes up and runs some CPU intensive jobs while sitting in a sleeve.