Need help diagnosing a headache

I've been getting headaches for quite a few years now late at night and early morning. They wake me up and are pounding in the back of my head, especially with positional changes like bending forward. First thought is maybe hypertension but my blood pressure is normal. I do not believe they are migraines and I know they are not cluster headaches because I suffer from those seasonally.

Background: I am 33 years old, smoke about 6 cigarettes a day, normal weight, under stress, rapid heartrate (probably stress and caffeine). I drink a fair amount of caffeine.

The headaches are worse if I oversleep. They occur at random frequencies but they tend to occur in groups of days. My stress level is consistent as is my caffeine intake. Only ideas I have is that it's from caffeine withdrawl or some type of blood pressure related headache.

Any ideas?

Thanks,
Darrin