Some owners of the new 4th-generation Apple TV are reporting that the devices are waking up at random times, resulting in the devices also (in some cases) turning on the HDTVs they're connected to. AppleInsider reported this morning that the issue has become the subject of a number of threads on Apple's support forums.
Once put to sleep, the Apple TV can wake up as soon as a minute later. When the HDTV has HDMI-CEC support allowing the Apple TV to turn on the TV, both of the devices can pop back on. Let's just hope you remembered to turn down the volume on the TV.
The issue can waste electricity when the TVs get turned on, and owners of plasma TVs even risk burn-in issues if the tvOS screensaver doesn't kick in. Possible workarounds at this time include closing some or all apps prior to putting the Apple TV to sleep, restarting the unit before putting it to sleep, or turning off ad tracking. If keeping the TV off is your main concern, HDMI-CEC can be disabled in the tvOS settings. There's no word on whether Apple is working on a fix for the issue.