A surprisingly popular form of nightly rest in the Middle Ages, mostly with members of State and highly paranoid soldiers. Sleeping at a 45 degree slant (head up) allowed someone in deep slumber to spring into action should anyone attempt to sneak into the bedroom in the middle of the night and murder him.

King Charlemagne is said to have slept in this manner.

In the sci-fi TV show Babylon 5, the Minbari are said to sleep at an angle for a different reason: To them, lying down is something that you only do when you are dead.

So, lying on a typical flat bed would be akin to a human sleeping in a coffin. However, there are angsty goth and vampiric types who think it is cool to do such a thing.

I find it somewhat interesting that kings and others slept in this way. It seems to me that sleeping in the usual, horizontal, fashion would allow one to quickly roll to the side if attacked. To be sure, it is easier to pin someone who is flat on a bed, but by the time you could do that, you could have stabbed them to death in any event, making the issue somewhat academic.
Additionally, a person leaning at an angle would require a less time-consuming movement to stab or hit with any sort of weapon, assuming that the assailant was standing. That would mean that it would be more dificult to dodge the blow, and a guard would have a second less to intervene.

As for the Minbari, just to clarify, I seem to recall that they saw sleeping horizontally as a bad idea not for purely psychological reasons, but because they had a superstition that it invited death. Thus, it would be akin to a human sleeping in a dangerous setting, rather than a merely morbid one. When Delenn, for spoiler-laden reasons, had occasion to sleep in a human bed, she seemed far from disgusted!

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