Why do stimulants make some people sleepy?

[ShawnD]

About a year ago, I started taking ephedrine on a somewhat regular basis (~3 times per week) to improve my mental and physical abilities. I take a fairly low dosage, only 40mg, and it works wonders most of the time. Some of the time, it does the exact opposite. During those bizarro times, I feel incredibly tired within a few minutes of taking it, and I get progressively more tired until I fall asleep. I sometimes sleep on peoples couches because of this, in a sitting position, in rooms that are well-lit and usually quite loud due to video games, TV, and several guys talking. I can wake up immediately and be ready to go if someone wakes me, but I can fall back sleep at the drop of a hat. If I'm at home when this happens, and I'm able to sleep without being disturbed, this weird ephedrine-sleep usually lasts about 6 hours.

I'm really curious why this happens, and why it seems so random. It has nothing to do with what time of day it is, how much sleep I had the night before, if I ate food or not, where I am, or who I'm with.

I used the generic term 'stimulants' in the title because a lot of stimulants seem to do this. I know a guy who can take a 200mg caffeine pill (amount of caffine in 6 cans of coca cola) and immediately go to sleep. One of my friends was on Ritalin as a kid, which is a type of amphetamine, and he claimed it made him tired. I assume these are somehow related.