Press and hold the TV/Control Center button on the Siri Remote to open Control Center
Select Sleep to turn off Apple TV
Press any button on the Siri Remote to turn it back on
If your Apple TV is connected to a TV that supports HDMI-CEC, turn off the TV to put Apple TV to sleep
Turn on the TV to wake Apple TV
Unplug the power cable to fully turn off Apple TV
Plug the power cable back in to turn it on again