I would think that this is related to breathing fully? I’ve found that when I tend towards feeling light-headed, it’s in poses where I am challenged to breathe deeply, and once I placed the focus on my breath, the light-headedness hasn’t really been an issue. Sometimes that means not going as far into a side bend or backbend if that last bit of range of motion causes me to hold my breath or turns my breathing ragged or shallow, but once I corrected the breathing, within a couple classes I was going even farther into the pose but able to continue breathing while in it! I wonder if you are not breathing as fully with your arms raised?
On a side note, I’ve found it very interesting that the only time I’ve seen someone actually pass out in a class was right at the beginning of class – in Half Moon pose. (And that’s one of those where my early tendency was to hold my breath…) It made it quite obvious that it wasn’t the build up of heat or overexertion as much as just plain not breathing that caused it.
