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.

My understanding is that you ought to take melatonin in small doses (0.3 or so) an hour or two before sleep in order to get tired, whereupon you sleep (normally, ish, unless you take too much and then you get crazy dreams or sleep paralysis). If I take it and go right to bed I always wake up still in the 'window' of its effect.

This thesis appears to be related or the underlying research:

https://d.lib.msu.edu/etd/51312

Indicates 2mg and 5mg were given along with the placebo. While these are common dosages available in the US, these are higher dosages than recommended by at least some sleep experts. The thesis doesn't seem to differentiate between the dosages when evaluating cognitive processes.

Now I want to evaluate my sleep stack against morning cognition.

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.

Even 200 mcg is enough to give me brain fog. I stick with 100 mcg unless I'm really having a hard time falling asleep.

Not sleeping impairs my morning cognition - a lot. Melatonin is part of the rotating stacks I use to mitigate insomnia.

My mornings have always been impaired, going back to pre-school. Meds don't make my mornings any worse than my baseline.

Could it just be dosage? Commonly found dosage is 10-20x what you'd actually secrete and IIRC absorption is actually quite good.

As a long term insomniac the head line result is not surprising. My overall experience with "sleep aides" is they all lead to impairments in morning cognition. The worst of which don't last just the morning... (seroquel, I'm pointing at you...) Melatonin is surprisingly powerful for a hormone / supplement. For me, it hits like a truck, and I find it strong enough that I had to stop taking it.

I'd be careful with the methodology here. There's been research that shows that most melatonin supplements are megadosed, with something like the "physiological dosage range" being 1 - 2 micrograms (ug.) So I'd hope the researchers got a trustworthy source for their tablets. It seems to me that with such a low dosage range you don't have much margin to avoid errors.

I think when it comes to sleep problems the worst possible place to start is with pills. Pills quickly lead to dependence and a lot of them either stop working or introduce so many side effects that there's little "therapeutic benefit." I think the right approach for insomnia is to focus on improving overall health. A lot of symptoms people go to the doctor for in that category are due to life-style factors. So if you maintain healthy habits you don't have to end up on 10 different pills just to function...

It's a slippery slope, trust me on that.