Hard to discuss this with only a short abstract. They don't even give a dose or formulation of the melatonin, and details like "We found no significant differences between the melatonin and sleep groups on any of our sleep measures." raise more questions than they answer.

The study is on "healthy young adults". How much improvement would you expect on someone who is already "healthy"? It's not like they are going to ascend and become superhuman.

Anecdote - I've used it dozens of times and I'm always foggy in the AM, so I avoid it if I have to be sharp the next morning. Perhaps it's a coincidence and this study is flawed, but I can believe it pretty easily.