Its most likely a sign of some kind of ill health. I'm not sure if its something to see a doctor about as I get it occasionally. For me it comes and goes and I know that I'm a little down when I get it.
I do however tend to self medicate this symptom by taking fish oil capsules. Seriously omega 3 fats from fish oil is JUST what my body NEEDS to get the furnace firing again.
Ohh and I DO suspect I have some thyroid issues stemming from toxic issues and I am going through a detoxification process. It just takes time and I'm not inclined to go to a doctor with thyroid issues even though they ARE serious as I haven't heard ANYTHING good about mainstream thyroid treatments.
Anthony