Well, I can see that you are concerned about your dilated pupils right? So, take it easy, generally speaking, it would be natural for our eyes to be dilated when we try to lie because that would make us nervous, which could give signal to the pupil muscle to react. Maybe sometimes our eyes would be dilated due to some horrible occasions but that is temporary and our eyes might be resulted from taking drugs. Just pay more attention to your eyes and if anything strange happens , try to visit a doctor.
Usually the pupils may be dilated because of the shocking or surprising symptom. However, when you have the eyes problems, like the infection in the eyes, you may have the dilated pupils. You'd better go to check your eyes fully to see whether there are the eyes problems for you. If you have, you'd better go to treat it as soon as possible.
Well, yes, it is possible for you to have dilated pupils if you lie. Generally speaking, you should know that dilated pupils can be caused by high blood pressure. So when you lie, there will have change in your body, leading to high blood pressure. In that way, you may suffer dilated pupils in your eyes. And your pupils can be bigger than the normal one. Also, anxiety can be lead to dilated pupils too. so just control your own emotion.