Why do I get sleepy in allergy season?

Kat Arney put John's question to Naked Scientist Chris Smith...

Chris - It's a difficult one this isn't it, because there's a number of things going on here. I know for a fact that when I get a very blocked nose and cloggy throat because of allergy, I don't sleep very well. And so, I think the part of this is that we don't get a good night sleep because you're continuously tossing and turning, sort of clearing your throat, trying to blow your nose and so on. Even when you're asleep you're still being disturbed in your sleep, so you're not getting restful sleep and I think that's part of the equation. The other thing is that how do people manage an allergy problem? Well they take antihistamines because most of that allergy response is the release of histamine in your mucus membranes - eyes, nose, other parts of your body - and the histamine then produces all the symptoms. It causes the itchiness because it winds up the nerve fibres that supply that part of the body. It also causes blood vessels to open up and so-called dilate and become leaky. So, you get swelling of the tissue, and this causes your nose to become all puffy. When you take antihistamines, these are molecules that look like histamine but can't activate the receptors that histamine normally goes on to. So, they block up the histamine effect. Unfortunately, your brain also uses histamine to talk between one set of nerve cells and another. Histamine plays a really important role in arousal and awakening. If you block the action of histamine in the brain then you feel very sleepy. The best antihistamine drugs unfortunately tend to be the ones that are not selective between what the brain needs to do with histamine and what the rest of the body needs to do with histamine. There are antihistamine drugs that can exclude themselves from the brain, but they're not really as good. So, by treating your hay fever or your allergies with an antihistamine, you're actually also sending yourself off to sleep. And I think some of the problem is probably going to be down to that.