Android - Why doesn't the broadcast receiver fire when the device goes to sleep?

Question

Android

So, I am writing an application that should do some actions when the tablet is connected to the docking station.
Declared in the manifest

<receiver android:name="MyBroadcastReceiver">
            <intent-filter>
                <action android:name="android.intent.action.DOCK_EVENT"/>
            </intent-filter>
        </receiver>
        <service android:name="PowerService"></service>

respectively, the receiver starts the service

public class MyBroadcastReceiver extends BroadcastReceiver {

  @Override
  public void onReceive(Context context, Intent intent) {
    // TODO Auto-generated method stub
    context.startService(new Intent(context, PowerService.class));
  }

}

I display a message in the service

public int onStartCommand(Intent intent, int flags, int startId) {
    IntentFilter ifilter = new IntentFilter(Intent.ACTION_DOCK_EVENT);
    dockIntent = registerReceiver(null, ifilter);
    Toast.makeText(this, "Старт сервиса...", Toast.LENGTH_LONG).show();
    if(dockIntent != null){
//бла-бла-бла

When the tablet is connected to usb, everything works well, there are no problems. When the tablet is disconnected from yusb, everything also works until the screen is off. After the tablet goes out and I try to connect the docking station, the screen lights up but the service does not start. What could be the reason?