The Myo armband automatically enters a battery-saving sleep mode when it is idle for a short period of time. It will exit sleep mode when you pick it up.
You can also turn Myo off completely by using the Turn Off button in the Armband Manager window of Myo Connect. This is great to use when travelling with your Myo armband!
In order to turn Myo back on, you must connect it to your computer or a wall charger through the micro USB cable.