In England we call these cramps. In pregnancy you call them the nighttime miseries! In the middle of the night I sometimes get a charley horse in my leg. According to Parents.com:
“If you’ve ever had a “charley horse,” then you’re already familiar with the kind of leg pain that many women encounter during pregnancy. No one is entirely sure why these sharp, lower-leg cramps crop up during the second and third trimesters; doctors theorize that it may have to do with the pressure of the uterus on certain nerves, fatigue, or decreased circulation in your legs from the pressure of the baby on blood vessels. In any case, you may be awakened with sudden painful spasms in your calf muscles (they most often occur at night). They aren’t fun, but they’re not usually cause for concern, either, unless they persist and are severe, in which case you should tell your healthcare provider.”
I seem to get them more often when I haven’t been drinking enough water, but maybe that’s just me. I have heard that wheatgerm lessens the frequency, but the cure is simply to stand up. It seems once the blood flows back to the area, you’re good to go… back to sleep.