Why Does A Person Have Night Time Leg Cramps? Mostly

Why does a person have night time leg cramps? Mostly in the upper thighs or outside of the lower limbs. So sudden and painful, that I must immediately jump out of bed and try to straighten out my limb. I tried eating 3 tablespoons of mustard right after the cramps, and that seems to ease the cramping. Am I missing some key element in my diet? Thank you. YYYY@YYYY