I've heard melatonin doesn't necessarily put you to sleep, per se, it just helps you regulate your cycle better. Never used it myself so I can't say for sure but anecdotal reports indicate that it's not the best sleep aid.

I used Phenibut the night before each of my practice exams and then my real MCAT. I was very pleased with the results. If you have any type of substance abuse in your personal or family history, though, STAY AWAY FROM PHENIBUT. I don't so I was fine, but you can get addicted to it very easily apparently. Phenibut is GABA with a phenyl group attached to get it to pass the blood-brain barrier more efficiently. You take 300-500mg 1-2 hours before bed, it helps you get some shut eye, and even the next morning you don't feel freaked out. It's better than other sleep aids because it doesn't make you groggy. (The Russians put it in their cosmonauts' kits in case they get anxiety in space instead of another anxiolytic/relaxant because the cosmonauts need to be able to do math and shit if there's an emergency like an engine fire. In other words, Phenibut doesn't impair cognition provided that you don't overuse it either by taking more than 500mg (less is more so see what your optimal dose is) or using it more than once a week, and that's why the Russians give it to their people. Doctors actually prescribe it in Russia, too, so I've heard.)

Disclaimer: I'm not a doctor, talk to your doctor, blah blah blah. (Don't sue me or do something stupid/dangerous.) Also NEVER drink alcohol while taking Phenibut (too much GABA) and don't take benadryl or anything else that makes you drowsy with Phenibut.

Since I took the test I don't use Phenibut anymore because its power scares me. But it was definitely helpful to get me to relax and sleep for my exam. Do not use it for the first time before your real exam: see how you react to it before at least 2 practice exams, playing with the dose to see the minimum effective dose.