How to Automatically Disable Wi-Fi On An Android Device When it goes into Sleep-Mode

There probably isn't any reason for your Android phone's Wi-fi connection to remain active while not in use as it continues to drain the battery.Though, disabling Wi-fi on devices with 3G / 4G connection, will only increase data consumption and will probably do no good to the battery life unless it's configured not to sync on the mobile connection.
Therefore, this can only be advised on a Wi-fi only Android device such as Tablet.

So having taken that into consideration, here's how to go about it.

1. Navigate to the Wi-fi settings
2. Select Menu and choose Advanced
3. Use the “Keep Wi-fi on during sleep” option.
4. Finally, choose between the three settings available i.e Always,
   Only when plugged in &
   Never.

Note »
*. ‘Always’ mode :In this mode,the 3g or 4g connection is not in use.
However, it's a big threat to the battery life.
*. ‘Never’ mode : In this format, the Wi-fi connection will be cut off once the Tablet goes into 'sleep mode'.
But note, it doesn't cut off immediately as it takes a few sec. to do so.