Plutarch Quaestiones Convivales 4.2.4

[4] And when I put in my advice, saying that it was as seasonable to discourse of thunder and lightning amidst our cups as it would be in a comedy to bring in engines to throw out lightning, the company agreed to set aside all other questions relating to the subject, and desired me only to proceed on this head, Why are men asleep never blasted with lightning? And I, though I knew I should get no great credit by proposing a cause whose reason was common to other things, said thus: Lightning is wonderfully piercing and subtile, partly because it rises from a very pure substance, and partly because by the swiftness of its motion it purges itself and throws off all gross earthy particles that are mixed with it. Nothing, says Democritus, is blasted with lightning, that cannot resist and stop the motion of the pure flame. Thus the close bodies, as brass, silver, and the like, which stop it, feel its force and are melted, because they resist; whilst rare, thin bodies, and such as are full of pores, are passed through and not hurted, as clothes or dry wood. It blasts green wood or grass, the moisture within them being seized and kindled by the flame. Now, if it is true that men asleep are never killed by lightning, from what we have proposed, and not from any thing else, we must endeavor to draw the cause. Now the bodies of those that are awake are stiffer and more apt to resist, all the parts being full of spirits; which as it were in a harp, distending and screwing up the organs of sense, makes the body of the animal firm, close, and compacted. But when men are asleep, the organs are let down, and the body becomes rare, lax, and loose; and the spirits failing, it hath abundance of pores, through which small sounds and smells do flow insensibly. For in that case, there is nothing that can resist, and by this resistance receive any sensible impression from any objects that are presented, much less from such as are so subtile and move so swiftly as lightning. Things that are weak Nature shields from harm, fencing them about with some hard thick covering; but those things that cannot be resisted do less harm to the bodies that yield than to those that oppose their force. Besides, those that are asleep are not startled at the thunder; they have no consternation upon them, which kills a great many that are no otherwise hurt, and we know that thousands die with the very fear of being killed. Even shepherds teach their sheep to run together into a flock when it thunders, for whilst they lie scattered they die with fear; and we see thousands fall, which have no marks of any stroke or fire about them, their souls (as it seems), like birds, flying out of their bodies at the fright. For many, as Euripides says, For certainly the hearing is a sense that is soonest and most vigorously wrought upon, and the fear that is caused by any astonishing noise raiseth the greatest commotion and disturbance in the body; from all which men asleep, because insensible, are secure. But those that are awake are oftentimes killed with fear before they are touched; and fear contracts and condenses the body, so that the stroke must be strong, because there is so considerable a resistance.