HTC One User Manual page 39

Your first week with your new phone 39
Sleep mode
Sleep mode saves battery power by putting HTC One into a low power
state while the display is off. It also stops accidental button presses when
HTC One is in your bag.
Switching to Sleep mode
Briefly press the POWER button to turn off the display and switch to Sleep
mode. HTC One also automatically goes into Sleep mode when it's left idle
for a while.
You can change the time before HTC One sleeps in the Display, gestures &
buttons (or Display & buttons) settings.
Waking up from Sleep mode
To wake HTC One up manually, press the POWER button. If your screen is
protected with a lock screen, you need to unlock it.
HTC One automatically wakes up when you have an incoming call.