The first is that an allergic reaction is like being sick. Normally, we become sick when we have an infection. The body fights it off, which takes lots of energy. Result: you get tired.
An allergy is the body mistakenly thinking that an ordinary substance such as a wheat protein or gluten, is an infection. The body is wrong, but it treats the substance in the same way as an infection. Result: you get tired, just like having the flu or a cold.
The other reason, and I know this happens in my case, is that allergies can affect your sleep. If you can’t sleep properly, you will feel a bit tired the first day, more the next, and eventually you will become chronically fatigued. Add this to the tiring effects of the body fighting the substance you are allergic to, and its not surprising that you get so tired.
Also, try to stay caught up on your sleep. Take naps if you can, go to bet earlier if you can. Avoid sleeping pills. They are addictive, and while the induce sleep, they do not provide true rest.
I hope that helps. If you have any other suggestions, please share them in the comments section.