The Real Reason Benadryl Makes You Tired

Benadryl is an over-the-counter antihistamine used to treat allergies and the common cold. Like many antihistamines, Benadryl is a safe and effective way to help relieve cold and allergy symptoms (via Live Science). Benadryl works by blocking your body's response to histamine — a naturally occurring compound that triggers an immune response to allergens. Unlike other allergy medications, like Allegra and Zyrtec, however, Benadryl can also cause drowsiness.

That's because Benadryl doesn't discriminate between which histamine receptors it blocks in the body. In addition to blocking histamine production in the immune system, Benadryl also blocks histamine receptors in the brain, which are responsible for regulating your sleep-wake cycle. The histamine levels in your brain decrease as you get ready for bed at night, which is what makes you feel tired and sleepy. Since Benadryl mimics this by blocking histamine production in the brain, it causes the same sedating effect, making you feel drowsy and tired. 

Since Benadryl is so effective at causing drowsiness, can it also be used as a sleep aid? That depends. While taking Benadryl may help you fall asleep initially, that doesn't mean that it will help you stay that way (via Shape). Benadryl only lasts a few hours before it starts to leave your system. It's also not recommended for use as a long-term sleep aid.

That's because continual and excessive use can actually cause the drug to lose its effectiveness. "Generally, its long-term effectiveness is minimal, and after four or more days of chronic use, it's debatable as to whether it has any effect as tolerance develops quickly," Dr. Christopher Winter, the author of "The Sleep Solution: Why Your Sleep Is Broken and How to Fix It," told Shape. If you're having consistent trouble falling and staying asleep, you should contact your doctor. Although Benadryl won't be of much help, your doctor may be able to find a more effective solution to your sleep problem.