There are environmental factors that can increase the onset of an attack. There are many factors that can contribute to seizures though it depends from person to person. Epilepsy is a seizure disorder and as such it can only be figured by multiple seizures. They usually will have you stay for a week or two for continued and constant monitoring. It's really the only way an EEG will catch it unless your lucky, I wasn't, for them to catch on a quick EEG.
I've been told by my son's doctor that unless he is having a seizure it will not show up on an EEG. I don't know about the factors that cause epilepsy but there are hundreds of reasons known and unkown for seizures.