Alternatives for EMG muscle sensors?
Although my answer goes against being electrical engineering in nature I have to say you are going about this all wrong. Even if you could somehow make a gadget how can you be sure that trying to measure tension in one muscle is even going to be the right one when the next time it may be in completely different area.

What you need to consider instead is a holistic approach. If you are having muscle seizures and cramps while you sleep be assured that is almost always caused by something going on in your life or physical self. It could be stress from any one of dozens of factors. It could be the condition and environment you put yourself through during the day and reflecting that in through the night. 

I would suggest that even trying to hook yourself up to a gadget that wakes you when these conditions happen will make things even worse. There will be added tension and stress due to the anticipation of being awakened and will certainly lead to less sleep and lower quality sleep when and if you do get to sleep. 

You may want to try some of these things to try to being more calm and reduced stress and tension in your being:


Try to reorganize your day - get up earlier - do your most energetic activities early and then by evening make sure there is a quiet wind down time for a couple of hours before trying to go to sleep.
Try learning yoga and take a class. The benefits to overall body stress relief can be amazing. 
Consider a treatment of whole body acupuncture. Even one session will show you what total relaxation can be like.