Hungry??

it might be what you're choosing to eat for lunch. Yogurt and granola is awesome and i love it, but I don't know if I would go with that for lunch. When you're eating fewer calories, it's easiest to stay fuller longer when you increase the volume of food. At least that's what I've found. I'd try having a really big salad with some chicken or other kind of protein, some avacado, and lots of colorful veggies! The protien and good fats will help keep you satisfied for longer
Maybe opt the yogurt and granola out as a snack or breakfast instead?

just my thoughts on that matter

For me, breakfast and lunch are my biggest meals (usually). I can't sleep very well if I eat too big of a dinner, even if I eat it well before bed time.

However, I have noticed that my need to feel "full" has greatly diminished. I now usually just eat until I am satisfied but not "full."

How many calories are you eating?

Just wondering......does your body get used to eating less for lunch?? I used to eat quite a bit for lunch, now I'm treating it more like it's a snack, so I can save my calories for breakfast and dinner. So recently instead of eating, say, a hoagie and chips for lunch, I've been eating yogurt and granola. But I find myself getting very hungry only an hour or so later, and then when that happens I get a killer headache from hunger. Will my body get used to this after a while??

You will adjust, your stomach shrinks very easily...as for headaches, that could be dehydration or some other issue.

Basically this is where people get the whole "multiple small meals over the day thing", it doesn't boost your metabolism, doesn't enact some magic weight loss wizardry, it curbs your hunger, which may keep you from overating..it's a discipline thing.

You will get used to it, hell if you fasted for 16 hours, then ate one large meal over the course of a couple hours you'd feel pretty good (and this is common for folks who intermittently fast).

Another thing that will happen is if you ate things that typically spiked your insulin (lots of sugar, simple carbs). Mind the granola even, look at things with typically high sugar, try raw honey instead, or almonds. Experiment with foods that satisfy you more.

Your body will adjust to eating less at particular times of the day, but that usually just means that you don't feel as hungry at that time. Symptoms such as lethargy and feeling weak may indicate that you need additional calories and nutrients, while a headache often indicates that you are beginning to dehydrate. Drink more water and see what happens.