What Causes Muscles In Shin Of Leg To Contract?

When ever I get still the muscles in my shin contract and make my foot jump up. This will happen over and over with any where from a 8 second to 15 second pause in between jerks. I can time it. It's very disturbing and is causing me a lack of sleep and rest. Lately it has been getting worse and is causing muscles in my legs to contract. Please help.