What about things like coffee
Gummies may break the fast. It might come down to what the melatonin is packaged in. Melatonin in pill form would be fine.
That said, getting good sleep is really important, so if the gummies helps you sleep, it might be worth it regardless of your fasting.
And black coffee is fine. Just don’t add creamers or sugars. (Artificial sugars should be fine, depending on who you ask and what kind they are.)