Supposing a vampire breaks into your room (supposing that said vampire didn't require permission to be given for said entry, and supposing that vampires were, in fact, real) and settles in for a snack. How long would the vamp need for a decent feed before it could retreat to the safety of the night?
According to physics students at the University of Leicester, around 6.4 minutes.
In this time, a hungry vampire could suck down 15 percent of an average human's five liters of blood from the carotid artery in the neck.
To calculate that fill-up time, the students took into account the average human blood pressure in arteries compared to air pressure. They also calculated how splitting the blood flow from the aorta, the body's main artery, into five other arteries, would affect the velocity of the blood.
The density of blood at room temperature also played into the calculations. Velocity, pressure and density were combined to gauge how fast and how much blood would come out of two half-millimeter puncture wounds (those are some very small fangs).
It's all very dependent on those set variables, though. The blood, for instance, could drain faster if the vampire had larger fangs, or applied suction to the wounds. The 15 percent measure also assumes the vampire doesn't actually want to hurt the victim. That percentage was chosen simply because it's the most blood that can be taken without affecting the victim's heart rate.
"In this investigation we found that it takes 6.4 minutes to drain 0.75 litres of blood from the human body, this seems fairly reasonable considering it takes less than an hour to give 0.47 litres of blood when you donate from a vein," the paper concludes.
"However this blood is coming from your arm and the blood pressure is lower here whereas ours is coming from the external carotid artery. To take it one step further we could take into account more than 15 percent of the blood being lost from the body and also the pressure if the vampire was sucking as well as drinking. This would reduce the time taken and make the process more efficient."
The paper was published in the University's Journal of Physics Special Topics, a publication that allows the students to flex their creativity by applying their knowledge to the weird and wonderful, while giving them practice at preparing scientific papers.