For some time I was eating one serving each of two high fiber cereals. . .
with a cup of milk on that. I would just about fall asleep, even though I always took a cup of some sort of caffeine with it.
Giant load of carbs PLUS milk, which has always put me to sleep equalled nappy-nap time for me.
I now eat a pair of corn tortillas, 3 eggwhites, about a cup of mixed cooked green things (usually arugula/spinach/kale, whole green onions, jalapeno pepper) and an ounce of goat cheese. This does not put me to sleep and I will not get hungry till almost mid afternoon usually.
If you are taking an acid reducer, like Zantac, stop. When you use zantac you can interfere with the uptake of b vitamins and coenzymeQ10, which can make you tired in deed. Before I weaned myself off of this stuff, I felt like I was half sleep walking. I feel much better now.