Why is my MP4 keep turning off by itself?  How can I fix this problem?

How to adjust the unit's sleep time?

Please refer to the picture above.
1. Press the MENU button to get in the Setup page (1st picture)
2. Press the RIGHT arrow to move down to the Power off option. (2nd picture)
3. Press the MENU button to get in the Power off page  (3rd picture)
4. Press the MENU button one more time, you will see the option panel to select the sleep time. (3rd picture)
5. Move the time selection by the LEFT or RIGHT arrow.
6. Press the MENU option when done.
7. To exit this screen, HOLD the MENU button down.