There are lots of reasons may cause dilated pupils. Did you drink too much last night? Or are you often drink alcohol? It will cause the dilate of pupils to your eyes. Or have take some drugs like ecstasy, amphetamines , cocaine, crack and meth etc. Or are you go somewhere that exposure to some kind of biological or chemical poison? It may also lead to pupil dilation. What's more, staying in a relatively dark area also causes the pupils to dilate.
The main cause of dilated pupils is the light. When you are in the dark environment, you pupils tend to be dilated to allow more light in. Many types of drugs and medicines are known to be a cause of dilated pupils. Besides, some kind of biological or chemical poison can also cause puils dilated. It is said that seizure disorders, including epilepsy and a concussion can cause dilated pupils. You should go to see a doctor to find out the exact reason of your dilated pupils.