Some people (including myself) on some occasions find that they get sleepy after lunch. In some cultures, they actually go home and go to bed, which they call a siesta.
At first, my guess is that after a full meal, blood is diverted to the process of digesting the meal and less blood to the brain, giving rise to sleepiness. However, this explanation is not always consistent. Because I have found that there were times when I was sleepy in the early afternoon even when I haven’t eaten.
Then I came across an alternative explanation in the Dr. Dement’s book The Promise of Sleep …
“In the early afternoon, between the two peaks of heightened clock-dependent alerting, the clock slacks off in its efforts to keep us awake. The result is post-prandial drowsiness, which most people wrongly attribute to the aftereffects of eating lunch. In reality, people are only feeling their accumulated sleep debt, unopposed by clock-dependent alerting.”
We accumulate sleep debt every hour that we are awake. The greater the sleep debt, the more sleepy. However, there are two clock-dependent alerting that fights this sleepiness — one when we first wake up and one around 4pm to 5pm.
Early afternoon is when this alerting is low to fight off this sleepiness of the accumulating sleep debt.