When dealing with anxiety it's logical that you would be tired for a couple reasons. If our mind is weighed down with worry and fear, we are going to be producing adrenaline and the hormone Cortisol as well...that will keep the mind and body at a heightened state. If the body is tense, your muscle are tense and for long periods of time...leaving us mentally and physically tired.
The other thing is...many don't sleep enough or sleep well enough. This is why exercise can play an important role in relieving the stress built up in our bodies, which will help clear our minds a bit.
The more we dwell on fearful thoughts, the more we keep ourselves revved up...the mind needs a break, our bodies need a break and in too many cases...it doesn't get a break because we focus on the fear and worry....that is enough to keep you wore out on a regular basis.
We could do ourselves a favor and quit looking for things to worry about and focus on and give the mind and body a break.