I don't usually look at interactions, because it's more important to establish an effect exists in the first place. Right now, I know that melatonin helps in getting to sleep, vitamin D impedes getting to sleep, high doses of magnesium citrate have ambiguous effects on sleep, alcohol usage seems to correlate with early bedtime (I forget about getting to sleep), and I speculate that Redshift/f.lux and masturbation help in getting to sleep but I haven't analyzed that experiment yet. I haven't looked at any interactions with nicotine or modafinil or anything, since I'd expect them to just independently make it harder to get to sleep.