A study published recently on the website of the British magazine New Scientist shows that post-meal drowsiness is caused by elevated blood sugar levels, which inhibit neurons in the human brain that maintain wakefulness and alertness.
Professor Denis Burdakov of the University of Manchester, who led the research, explained: "This finding aligns with basic biological principles, as post-meal drowsiness may be the body's way of telling us to relax and conserve energy."
Burdakov's research team studied a group of brain cells called "Orexin" neurons. Located in the hypothalamus, Orexin neurons produce Orexin protein, which plays a crucial role in maintaining normal wakefulness. These neurons are inactive at night, and if they malfunction, they can cause narcolepsy, a condition characterized by excessive daytime sleepiness.
Previously, experts knew that blood sugar could inhibit Orexin neurons, but they were uncertain about the sensitivity of these neurons. Burdakov's research team exposed Orexin neurons to subtle changes in blood sugar levels, similar to those that occur after eating and when fasting, and then measured their firing rates.
Burdakov said: "We found that a slight increase in blood sugar from a normal meal is enough to shut down the activity of Orexin neurons." He explained that this means the neurons are more active when blood sugar is low, so people feel sleepy after eating and find it hard to sleep when they are hungry.