The PM pill you took at 10pm is still in your brain at breakfast
Brain scans 12 hours after a nighttime dose found about half of the targeted receptors still blocked.
That heavy head at 9am may not be a bad night. It may be the pill you took to avoid one.
Diphenhydramine is the drowsy ingredient in most PM pain relievers and many drugstore sleep aids. It works by blocking histamine, a brain chemical that keeps you awake.
Histamine neurons fire all day from a small cluster deep in the brain. Block their receptors, called H1 receptors, and alertness drops.
Newer allergy pills were designed to stay out of the brain. Diphenhydramine crosses right in, which is exactly why it makes you drowsy.
That is the point at 10pm. The problem is how long the block lasts.
Researchers at Tohoku University in Japan gave volunteers diphenhydramine at night and scanned their brains with PET the next morning, about 12 hours later. The results were published in the Journal of Clinical Psychopharmacology in 2010.
About half of the brain’s H1 receptors were still blocked. The team notes that earlier work tied that level of blockade to strong drowsiness and impaired brain function.
In other words, the pill was still working at breakfast. The sleepiness you bought for the night was partly delivered the next morning.
That leftover block is the likely source of the fog. Slow thinking, a thick head, a morning that takes two hours to start.
This was a brain-imaging study, not a sleep study. It shows where the drug still was the next morning, and the researchers’ own advice was to avoid it the night before driving or an important exam.
Check the active ingredients box. Doxylamine, the other common drugstore sleep ingredient, belongs to the same drug class and was not tested here.
The pill you take in the last hour is still in your brain at breakfast. Half the receptors, twelve hours later.
This is about the sedating kind used as a sleep aid. If you take an antihistamine for allergies, or a doctor told you to take this one, talk to them before you change anything.
