I had this too and it was usually really bad on those really hot days where I was a bit dehydrated and had been on my feet all day. I also read it was a result of a lack of magnesium and essential salts which would make sense why it was worse on those days I was a bit dehydrated.

I used to get DH to massage my legs with a peppermint cooling leg gel just before I wanted to go to sleep and it would relax them enough so I could drop off.

HTH!