You say you're sleeping more, but how's the quality of your sleep? Some people who work the night shift find that the sleep they get is not very good. Even if you're in bed for 8 hours, if the sleep you get is choppy, that is, you're not getting enough of the deeper stages of sleep, you could have symptoms such as you're describing. Maybe the night shift isn't the best for you.
Like Andrea said, see your doctor to make sure there's not something else going on. If everything checks out okay, consider finding a job with daytime hours.