What can I do to stop my legs cramping during the night?

No one really knows why leg cramps are more common in pregnancy, but most likely it is because the muscles are tired from carrying the extra weight. It may also be aggravated by the pressure of the uterus on the blood vessels from the legs to the heart.
How to reduce leg cramping,
• Avoid standing or sitting with legs crossed for long periods.
• Stretch your calf muscles regularly during the day and several times before bed.
• Take a walk daily, unless your Doctor/midwife has advised you not to.
• Avoid getting too tired.
• Stay hydrated, drink plenty water.
• Try a warm bath before bed to relax the muscles.
(There is some evidence that taking Magnesium supplement may help some women, however check with your Doctor first
It is also believed that taking a calcium supplement may improve cramps, so far there is no evidence to support this.)