spasms and weird waves of darkness during meditation

Hello everyone. I have been practicing meditation for a while now and meditate pretty much everyday. Usually when I meditate I get a sense of calmness and peace and heightened awareness to everything that is happening around me. Recently though, while in meditation there are subtle spasms in my back and stomach. has anyone else experienced this before or know what it is? Also when I meditate with my eyes open, the deeper I go into the meditation waves of darkness will begin to flow across my eyes until it is near pitch black. Then the light will return and the cycle will begin again. What is this? Is it normal? Does it have to do wth the meditation or something on the physical level? Thank you.

-Wanders

The eye is normal- I think it's called retinal fatigue. If you move your eyeballs and focus on a different spot I'll go back, but if you keep focusing on just one spot It'll happen again. As to the spasms, not sure, since I don't know what position you're in or if it's little twitches (which are normal for staying aware as your body starts going to sleep.)

ps. Retinal fatigue- "..when the afterimage of an object cancels the stimulus of the object on the retina."
http://www.scientificpsychic.com/graphics/

Thanks. For the spasms it doesnt matter what position I'm in wether sitting down with legs crossed, in a chair, or lying down. And yes they are twitches so it must be my body going to sleep while I'm still aware.