Waking up by light is the most natural method of waking up. Many people have unlearned this method so it might take some days before it will work. Even if you do not wake up by it your body may still prepare for wakeup. Your eyes can detect morning light even when they are closed. Especially in the winter time and when it is really dark in your room in the morning. Always use a sound wakeup as a backup a few minutes later but let the body prepare for wakeup only by light.