As a general rule of thumb, I don't set it for the regular 15 minute increments, because it probably won't go off. Today, I set it for 10:37 and it went off at 10:46! 😠
How do you know it wasn't going off if you weren't awake? I've had situations before where mine went off but my brain incorporated it into the dream I was having for a little while before waking me up.
Maybe it has a setting to gradually start the noise as well? It wouldn't make a lot of sense for it to introduce the light gradually while also blasting you with the sound.