The old post lunch slump
"Why do I feel sleepy after lunch?" Melanie Jans-Singh has been looking into exactly this...
Turns out that sleepiness after lunch isn’t necessarily related to whether you had a high carb pasta dish or a light salad (although a big lunch might exacerbate it). Some people experience a period of afternoon doziness regardless of whether they’ve even had lunch, and it’s related to your body clock.
From the moment you wake up, your body starts, little by little, wanting to go back to sleep; and the later in the day it gets, the more this “sleep pressure” builds up. You also have an oscillating, roughly 24 hour, pattern of wakefulness; your alertness peaks around the middle of the day, and drops as the afternoon progresses and sleep pressure builds. It’s the growing difference between these two signals that can provoke an afternoon slump.