Is there any research on physiology of deep and long meditative states (i.e. days and weeks of sitting)?

I keep hearing about Theravada monks who are able to stay in deep meditative states for days and weeks on end. While I do not [yet] question ability to stay in meditation for that long, what interests me is the physiological side of things: how human beings are able to spend that much time meditating without dying of dehydration or having some issues due to blocked blood flow etc. Has anyone seen any research on topic or can offer a proper search query for Google scholar?