HTC
When you have connections such as Wi-Fi on while the phone is in sleep mode, some apps may still continue to use the battery. Starting from Android 6.0, battery optimization is enabled for all apps by default which helps prolong battery standby time when you’re not using the phone. The phone needs to be sitting idle for an extended period of time before battery optimization takes effect.
If there are apps that you don’t want battery optimization enabled, you can turn the feature off in those apps.
Open up Phone Settings > Power > Battery optimization >Not optimized > All apps to see the complete list of apps.
To turn off battery optimization, tap the Pomodoro app > Don’t optimize > Done.
To filter the list and see which apps have battery optimization disabled, tap All apps > Not optimized.