One of the reasons for that the pupils become bigger is that the light rays in the environment is too weak, so the pupils get larger so as to allow more light rays to come into the eyes. Maybe this is the reason for your pupils getting bigger when you woke up. It is normal, and you do not have to worry about it. But there is another reason for bigger pupils, that is, over-fatigue, high pressure, imbalanced diet, and so on. If so, you should pay attention to have a healthy and regular diet, and do not overuse your eyes without rest. Keep in a relaxed mood, do not get anxious. Eat more vegetables and fruits, as well as dairy products and fish.
I know that our pupils will become bigger when we are very excited. Under such circumstances, it may be caused by the reaction of our body. Beside this, our pupil will become bigger when we saw what we want when we are shopping. But as to you, you just wake up, i think it may be the reflection of black things, because sometimes our eyes will act like a mirror, this will make our eyes look more darker. Or you are just in a dark room, you pupil need to dilate to see things clearly.