shycloud,
It is bec yr blood circulation.
According to pre-natal lesson, we should not stay in one position for too long. If u sit too long and suddenly stand up, u will get this problem i guess. When we change position, we are suppose to be slow. Dun do it too fast.
In addition to avoid numbness, leg cramps etc, pls take 1000-1200mg of calcium per day. My gynae said it can avoid leg cramp.
In addition, do massage on yr legs b4 sleeping.