Why am I tired all the time?

For some reason, I am ALWAYS tired. I live a mostly healthy lifestyle, including plenty of good exercise (military), good eating habits, good brain workouts (reading, brain teasers, etc.), and about 6-8 hours of sleep a night. I work 12 hour shifts, mainly consisting of keeping track of aircraft and answering phones, 5 days a week with 2 days off. I take multi-vitamins daily, along with OMEGA-3 vitamins and Vitamin D. Ever since I PCS'd to Guam, I've been tired just about non-stop. There has not been a day that I can remember in the past year and half where I haven't had energy to do stuff all day, and I've been tired at least half the day. What's going on here? Any suggestions?