It depends on what is causing the seizures and your brain activity when you are either sleep deprived or sleeping. When I had my SD EEG, they told me to try to fall asleep but it was ok if I did not. The room was very quiet and comfortable, they put warm blankets on me and I was very tired. I did not think that I would fall asleep but I did for just a little bit. And that is when I had a seizure. I am not sure if they would have been able to diagnose me properly if I did not have a seizure at that time, i forgot to ask that question.
It was mostly to confirm that I do have partial seizures and the next step was to have CT scan, MRI etc to see if there is something in there causing it. That could be a tumor, scar tissue or what ever. So far after all of those tests they can not see what is causing my problem. Very frustrating.