Retrieves remaining sleep time, if any

        Returns:
            int or NoneType: Number of seconds left in timer. If there is no
                sleep timer currently set it will return None.

def get_sleep_timer(self):
        """Retrieves remaining sleep time, if any

        Returns:
            int or NoneType: Number of seconds left in timer. If there is no
                sleep timer currently set it will return None.
        """
        resp = self.avTransport.GetRemainingSleepTimerDuration([
            ('InstanceID', 0),
        ])
        if resp['RemainingSleepTimerDuration']:
            times = resp['RemainingSleepTimerDuration'].split(':')
            return (int(times[0]) * 3600 +
                    int(times[1]) * 60 +
                    int(times[2]))
        else:
            return None