Everyones symptoms are a little different so I am not sure to how many people this observation will apply...but...I think one of those that might have been able to relate was Dracula.
I am at my best (for what that is worth) in the middle of the night and at my worst in the middle of the day. As the sun rises I start to feel weak and lethargic and as it sets I regain some energy. I have become a little nocturnal though I have not gone to the extremes of popping into a coffin at sunrise and rising at sunset.
Then there is the possible body chemistry connection. Note that what follows is purely a fanciful conjecture as I am not going to experiment! Body chemistry plays a role in how well an MS damaged body transmits signals across the nervous system. Exertion changes that chemistry to the determent of movement. Heat is part of that reaction as heat is evil for most MS sufferers but I understand that the body chemistry also changes when your muscles work (which explains my symptoms getting worse when I exert myself as I certainly don't work hard enough to raise my body temperature! A drug called Fampyra (Ampyra in the states) changes the body chemistry in a positive way (minor unfortunately).
So here is the fanciful conjecture. What impact did the fresh blood have on Dracula's body chemistry and therefore on his MS symptoms? It would have had to have had some effect. As I recall there was no substitute for the real thing either. Has to be fresh and warm and human. Not exactly something that the National Institute of Health or the National Health Service is going to research is it?