Some medications such as antihistamines lower allergy symptoms by reducing or blocking histamines—chemicals the body releases in response to an allergic trigger. 

Histamines are naturally involved in your sleep cycle.  Histamines cause stuffy nose, itchy, watery eyes, and skin rash that are the characteristics of an allergic reaction.  While antihistamine

s can reduce these symptoms, they also can cause unpleasant side effects.  Antihistamines – even those that claim to be “non-drowsy” – if taken in high enough doses may cause drowsiness.

Newer generation of allergy medicine such as cetirizine (Zyrtec), levocetirizine (Xyzal), and desloratadine (Clarinex), or older drugs, including hydroxyzine (Atarax) and pheniramine (Avil), have fewer antihistamine side effects than older, first generation antihistmines.

Results of the study named: “Assessment of the Effects of Antihistamine Drugs on Mood, Sleep Quality, Sleepiness, and Dream Anxiety,” were published in the International Journal of Psychiatry in Clinical Practice[1] in 2014.

The results showed that all of the drugs studied—not just the first–generation antihistamines—were associated with increased daytime sleepiness.  Furthermore, patients receiving Zyrtec reported higher depression, anxiety, and fatigue scores than those who received the other newer-generation drugs.

 

Sources: 

The Myth of Non-Drowsy Allergy Medication