Drowsiness occurs because something is imposing a load on your body's energy supply.
In my opinion, the #1 most-likely suspect is your digestive system.
When I was studying nutrition during my twenties, it came as quite a surprise to me how much energy the body uses in digestion. I can't remember the estimate, but it's huge.
In fact, the way you eat determines the size of your digestive organs vs the size of other vital organs like the heart and brain.
I specifically recall reading Enzyme Nutrition by Edward Howell. He offers data on rodents that show that, for example, eating "dead" food caused the rodents to grow their pancreas 5x bigger to handle the extra load, and it caused their brains to be 25% smaller.
After all, the body only has so much energy to parcel out.
So what is this "dead" food?
Well, all food was once living right? In that living state, it contained the enzymes required for its' own digestion. In other words, it is going to help you digest it, decreasing the workload on your digestive system.
Therein lies the case for eating things raw or with minimal cooking. There are, of course, lots of fine points and exceptions here. Don't just go out and start eating raw eggs from Wal-Mart.
Nuts and seeds are an exception to this, since they are designed to hibernate indefinitely until planted. This is why we (as a family) soak our nuts and our grains before eating/processing them.
If you struggle with drowsiness, I recommend that you make time to educate yourself on this as soon as possible. Start here.