Hey gwern, as you predicted I didn't have enough data to learn anything... and I didn't have time to do it longer. I considered repeating it, but now I'm scared off melatonin until I learn more about how it works. Dr. Ray Peat theorizes that it might have some negative health effects by inhibiting oxidative metabolism:

http://www.google.com/cse?cx=005233684413389937395%3Ad5qfhqsz7oo&ie=UTF-8&q=melatonin#gsc.tab=0&gsc.q=melatonin&gsc.page=1

Also, anecdotally I don't really see a huge benefit to melatonin. Even small doses (0.75mg) seem to make me slightly groggy when I wake 7.5 hours later. I may have unusually slow melatonin metabolism, as I have the "slow caffeine metabolizer" P450 CYP1A2 variant, the same enzyme responsible for clearing melatonin.