Why does my leg seize up?

This happens occasionally, usually at night before falling asleep, my leg will seize up. Both my calf and hamstring become sold as a rock and my tendons behind my knee are pulled so tight it feels like they're going to snap. It usually lasts about a minute or two but the last time it lasted for almost 10 minutes. I've​ tried googling this and found nothing as far as other people experiencing the same problem. Has anyone ever experienced this or know what causes it?