Is it possible for the government to prevent defendants from pretending to be insane in order to escape time in prison?

The other day I could not come up with a topic, however today in the computer lab I decided my topic would be about the insanity defense and ways to constrict it. There was a case dealing with the case of Harvey Milk and his assassination in 1978 by a man who claimed insanity. The case was that he normally ate healthy and was seen that day eating a twinky so he was in a mentally ill state that day. I am planning to use examples like these along with different methods of constricting this abused law to form my research paper.