When you are sniffling, sneezing and generally feeling poorly, you often don’t sleep well. But you might not make the connection between daytime allergy symptoms and a lack of sleep at night. And you might not realize that the combination can cause fatigue and other health issues. Also, an allergic reaction can release chemicals in your body that cause fatigue. If you have allergies, you might use over-the-counter (OTC) medications to deal with symptoms. But this could cause more problems. OTC medications, such as decongestants and some antihistamines, can disrupt sleep. There are many things that can cause fatigue.