Side effects - Stiff left shoulder ... ?

[Synesthesiac]

Not sure where to put this... but..

Of late swims noticed that when he's having difficulty sleeping that he's sleeping in a very odd position. His left shoulder is tense, and pushed upwards, as if his left side is permanantly shrugging. After noticing this he can relax it and return to normal position, but within a few minutes of not thinking about it its usually raised again and tense, so then he has to relax it, and the cycle keeps going over and over. Swims been using various CNS stimulants the previous months (MDPV mainly), and also benzo's to avoid bad crashes. Neither in addictive or particularly high amounts, 1-3 times a week. Swim suffered no withdrawals when he did nothing for a week a while ago, so he doesn't think its withdrawal effects. But it seems to be happening more and more recently, and makes relaxing and sleeping really hard.

Any ideas what could cause this? Is it a particular condition? Or just a general side effect of the Benzo's and Stimulants?