What is this dull ache in my leg?

From the time I was a small child I will occasionally get this weird ache in my leg or legs – all the way up and down, but more concentrated in my lower leg. It is dull and lasts for an hour or more, sometimes waning in and out. It kind of feels like it is in my bones, if that makes sense. It does not feel like a strain or a muscle injury. It seems to occur most often at night, when I am lying in bed.

Any ideas?