Turning on screen programmatically

Note from author: I wrote this back in 2012. I don't know if it works anymore. Be sure to check out the other more recent answers.

Amir's answer got me close, but you need the ACQUIRE_CAUSES_WAKEUP flag at least (Building against Android 2.3.3).

WakeLock screenLock = ((PowerManager)getSystemService(POWER_SERVICE)).newWakeLock(
     PowerManager.SCREEN_BRIGHT_WAKE_LOCK | PowerManager.ACQUIRE_CAUSES_WAKEUP, "TAG");
screenLock.acquire();

//later
screenLock.release();

This is very popular question but the accepted answer now is outdated.

Below is latest way to Turn On Screen OR wake up your device screen from an activity:

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O_MR1) {
    this.setTurnScreenOn(true);
} else {
    final Window window = getWindow();
    window.addFlags(WindowManager.LayoutParams.FLAG_TURN_SCREEN_ON);
}

Use WindowManager.LayoutParams.FLAG_TURN_SCREEN_ON but FLAG_TURN_SCREEN_ON flag has been deprecated in API level 27 so you can use Activity.setTurnScreenOn(true) from API level 27 onward.

In your main activity's OnCreate() write following code:

((PowerManager)getSystemService(POWER_SERVICE)).newWakeLock(PowerManager.SCREEN_DIM_WAKE_LOCK, "TAG").acquire();

It causes the device to wake up.

Do not forget disableKeyguard() to unlock the device...