Can I use "the" instead of "that" of second?
Native speaker’s answer
Rebecca
Yes, you can use "the" here instead of "that". In many cases, "the" and "that" can be interchangeable, however, "that" is often more direct and specific than "the", especially when directing someone's attention to something specific. In this context, "melatonin" is referred to more generally, and so it would be perfectly fine to say "the melatonin".