Could be mineral imbalance, parasites, toxic metals, some sort of allergy or intolerance or deficiency. Alcohol, dehydration, all these things! It very much depends on your lifestyle and other symptoms as to cause.
I have it too, where from who knows, how have I reduced it?
Proper diet, magnesium taurate (magnesium is often deficient) and Iodine supplementation.
They're actually quite common but the causes vary massively.