It could be stress and tiredness due to change in your routines. Also you need to improve your diet, take good meals and complete nutrient diet. That make you full of energy for the whole day.
You might try meditation -- it's even better than a nap. But the above is correct as well, it sounds like whatever you're eating is wearing off when you still need it. Many people say a 15 minute nap is a great thing to do. When I was in college I napped at the library for a few minutes when I got tired. But is what you're suffering depression, or just fatigue and feeling down? Not the same thing. Also, do you exercise?