It's recommended that you don't switch the POWER off to the box - i.e. don't turn off the switch on the back of the box, or the socket on the wall.
You can put the box into standby though, by pressing the power button on the remote. This will ensure it's using as little power as possible when you're not using it. If you don't press the power button then it'll put itself into standby by default, and it'll also go into deep sleep overnight to save even more power.
The box will automatically wake itself up to update the software on it, and to performance self-maintenance tasks. This is why it's recommended that you don't turn off the power - it'll ensure the box is running as best it can.
