I did a 3 day fast as an inpatient, to rule out an insulin-secreting tumour. I actually felt much better, but will say that I had an emotional crash afterwards, although I'm not sure how much of that can be put down to hospital insomnia/witnessing upsetting situations. They were also very concerned with taking my temperature and blood pressure at regular intervals (and of course blood glucose), so maybe wise to take those precautions.