It has never been physical as to why I have broke a fast. It was always an "excuse" being a valid excuse or not.. that's what has always caused me to break my extended fasts to soon. It's just something you push through, you have to be in control.
Working, or laying in bed, strenuous or not. You know your body, you have to listen to it. If you feel you are putting yourself into danger by not eating, then that's the choice you have to make.