Melatonin also has been found to have a stimulating effect on the glymphatic system, though we need better trials on that.
I take a mix of L-Theanine and Magnesium Threonate before bed and sleep way better now (feel more rested with fewer hours of sleep, and the FitBit confirms I get more deep and REM sleep).
Personally I didn’t have good luck with melatonin. It puts me to sleep fine, but I tend to wake up exhausted after ~4 hours and unable to fall asleep again.
I tried it because I'd been getting 4-5 hours of broken sleep due to peripheral neuropathy from a congenital spinal disorder for the past four or five years, and I finally feel like I'm healing. It's wild being able to fall asleep and stay asleep.
The main downside is that the effects are small - most of the research is in strength and endurance training and it seems like at most there's a 5-10% increase in lifting endurance (IIRC, it's been a while since I researched this).
It's one of a very few supplements that I personally feel should be generally taken by most people (unless you eat a lot of red meat).
The others are omega 3 (unless you eat fish several times a week) and possibly magnesium as the western diet tends to be low in that.
If you want to read more about effects of creatine on the brain this is a good paper:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7916590/
The consensus seems to be that supplementation of creatine is less clearly linked to increases in the brain than it is for muscle (although there have only been limited studies done), however, if you've ever suffered from depression, sleep deprivation or other neurological issues it's likely to be beneficial. For people without those issues, there's less evidence yet, but they would still get the benefit of more energy overall if not specifically more cognitive energy.
At least assuming 20ng/mL is a reasonable minimum. Some people suggest that’s a little low, but there’s real issues with excess vitamin D.
I don't know your dosage and use case but under creatine loading (5+ grams/die), water weight and bloating becomes a very, very noticeable side effect (in responders).
It's hard to know where to go for the right kind of supplement. I've taken it as an exercise supplement in the past with no seeming downsides, but in a few hours search, I couldn't find a supply that doesn't have dozens to hundreds of these complaints against it.
I'd put any one of those ahead of "creatine made me go bald".
From everything I've read, creatine is as close to being a "free benefit" as it gets. I didn't experience any overt improvement just from taking it, as in, no extra energy or motivation. However, I'd stalled on all of my exercises for about a year, and after a week of "loading" 10g/day, I started back and instantly hit my max reps in a single set. I didn't want to hurt myself and was scared of DOMS, so I waited another week before trying again (I had a single day of soreness). After a light warmup, I immediately did it again. The results of that have been highly motivating, and I've remained excited to push myself.
The only side effect is that I enjoy my tea slightly less. Creatine doesn't have a flavor, but it makes everything taste 20% less delicious, somehow.
other then that, creatine is considered the only supplement (protein not considered a supplement) that has virtually no side effects and known gain.
caffeine is behind that with some side effects.
beyond that, most other supplements havent been either to have proven benefits and proven no side effects
They went with the premise of "a little a day seems to help, what happens if we feed ridiculous quantities once". The sort of medical research that appeals to the engineer in me and something you can only really do with compounds that are already known to be unlikely to kill your patient if you exceed the usual dose. Also I strongly suspect they did this to students given the age range.
There's more literature out there than this. That paper's references are probably a reasonable start point. But as creatine is very cheap and has been widely consumed by athletes for ages now it's about as low risk a gamble as any out there.