The funny thing is, it doesn't really matter if I was busy working on hard technical challenges all day, or just reading articles/browsing the web; my brain shuts down the same time regardless.
The Spanish have siestas.
There is nothing wrong with you - this is human nature. It's in the scientific literature, TV commercials for energy bars, and even some European cultural identities.
But you know, when someone gives you a 30 minute break, you take it!
https://www.google.com/search?safe=active&site=&tbm=isch&sou...
I was very confused for a bit :)
Another popular way to sleep is to stretch out atop the seat and handlebars of one's e-bike (electric moped). Must be more comfortable than it looks.
Living in Southern Europe and working from home, my meal times vary but they'd be considered late for US standards (with the exception of breakfast). A late lunch (around 3PM) seems to do away with the mid-afternoon shutdown.
Also, at the start of the year, I changed from a healthy-leaning diet to one that consists almost entirely of muesli, lean meat, brown rice, fruits & veggies, eggs, trail mix, rice cakes, water, juice, and things of the like. I even switched to wine (instead of beer), whenever possible.
It's your body's natural response to having been fed - conserve energy until the next hunt.
I think it's more likely to be due to the energy it takes to break down food (meaning you have less free energy) that causes people to feel sluggish after a large meal.
Try skipping carbs at lunch, stick with protein+fat, see if that makes a difference.
It took a few months before I figured out what my portions were, since I don't feel full until about 15-20 minutes after I am. By the time I knew how much I needed to eat in one sitting, I'd become used to eating that amount, so it was a breeze.
I do eat whatever I want, with the exception of processed foods. If I can't make it at home with whole ingredients, I'll likely not eat it. I do cheat every so often, but not much, and not a large amount (I love a box of Raisinettes during a movie.)
As for not being able to think straight after lunch, I used to get that way when I'd eat too much. I used to love feeling full, but it would definitely affect brain function. Now, I get ill when I overeat. I also try to eat a high protein, high fiber breakfast, because it seems to get me further through the day than anything else (I eat a small late lunch).
I read plenty of literature on estimating the dietary recommended portion sizes in food, but nothing on how much food I personally should eat in a sitting.
As I say, it did take a few months. I also needed to eat a little slower - I was a fast eater of big meals. Mostly just experimentation and willpower - you have to get used to waiting to feel full. I go by the two fist rule: your stomach's about as big as your two fists put together.
My family also likes to go out to eat a lot. I ask for a box as soon as the food comes, and put half of the food in it. I used to finish the other half, but now I'll usually end up adding a bit to the to-go, and making two meals out of that later. If I'm by myself, I usually just order an appetizer (I do find myself eating more of just one type of dish per meal). If I go with my family, I'm ordering a meal, because, hey, free food!