for me, it comes and goes...BUT I'm beginning to think it mite be connected with getting even a little bit de-hydrated. For me, that is. Try and make that sort of connection, and if it is...just glurg more water than you thot you ever needed for a few days and then find your 'balance'.
Can't hurt to try? - j

PS at times it can get bad - so bad that it wakes up the DH? Not often mind you, but it's at those times it hits home that what I've got 'ISN'T' normal.