Airlines set their cabin pressure to 8000'. Everyone on board is breathing the same level of oxygen that they would if they were standing on top of an 8000' mountain. The density of oxygen at that altitude is much less than it is at sea level. Sooo....when you breathe that air it has the same effect as drinking alcohol, or cough syrup/antihistamines for that matter. Whereas those things block oxygen from grabbing onto your red blood cells, reduced density of oxygen means, well, less O2. That's a reason why so many people seem to sleep on planes. Giving kids medicine ups the ante, certainly, but it really isn't necessary.