I'm sure someone has anyway suggested melatonin? I usually take 1 or 2 gravol or benadryl to pass out after I'm done, but that's not natural I guess. Oh, and ear plugs, always. Hmmm... I guess you could try meditation? I'm not sure how successful you'll be with tricking your mind into sleeping after amphetamine. Honestly, if melatonin or other sleep aids aren't an option your best bet is probaby to just relax, put some ear plugs in, try and get comfy and let it run it's course. Depending on how long since I last dosed, I can't usually fully fall asleep but I can go in and out of sleep, teetering on the edge and usually feel refreshed after.
Sorry... this was probably totally unhelpful. Melatonin is pretty natural, we already produce it and it doesn't suppress your respiratory system at all.
Honestly, after I've just binged on speed for however long, it's all the same to me. Natural or not, the best thing I can do for myself is to get a real sleep.
Good luck