Lead poisoning
Because Van Gogh used lead based paints there are some who believe he suffered from lead poisoning from nibbling at paint chips. It was also noted by Dr. Peyron that during his attacks Van Gogh tried to poison himself by swallowing paint or drinking kerosene. One of the symptoms of lead poisoning is swelling of the retinas which can cause one to see light in circles like halos around objects. This can be seen in paintings like The Starry Night.