In Sleep mode, WiFi feature is still running. To save battery, switch off the WiFi feature before setting the device to Sleep mode. 1. Tap on ?Settings?. 2. Under ?Wireless & Network?, tap on ?WiFi?. 3. Tap the menu button at the top right corner. 4. Tap on ?Advance?. 5. Tap on 'Keep WiFi on during sleep' 6. Tap on 'Only when Plugged in'.