Follow the steps bellow to turn the Sleep Timer on and off:
On
Hit the Settings button on your remote.
Select the Sleep Timer option.
Select the duration of the Sleep Timer on the right-hand side of the screen.
To check if the sleep timer was set, from the home screen, select the Settings Icon on the Fire TV remote. You'll see a countdown has been activated on the right-hand side of the screen.
Off
Hit the Settings button on your remote.
Select the Sleep Timer option.
You'll see a timer and OFF in the lower right hand corner of the screen.
From the home screen, double check it is now off by selecting the Settings Icon on the Fire TV remote. On the right-hand side of the screen, you'll see the sleep timer icon, but this time there should be no countdown timer.