Next, the same experiment could be done with individuals who would be more likely to use melatonin supplements, or even just a broader group than "healthy young adults". I feel that the starting point of the experiment is a bit flawed since there's a high chance these healthy young adults don't have the problem the supplements are designed for. I'm not surprised about "We found no significant differences between the melatonin and sleep groups on any of our sleep measures" for these people.

Personal experience: I have mild insomnia* and I sometimes take melatonin. It generally does help, but I can be groggy in the morning, if I took it too late in the evening. Happens sometimes. But for me, it's so much better to be sometimes a bit groggy for a short while, compared to being completely exhausted from sleep deprivation all day.

*I was a startup founder for 10 years and practically did not sleep during that time. Even years after that life, it feels like my body is not responding to melatonin that much. Or my body is not secreting enough itself. Who knows.