For me it's not that Isaac Arthur is monotone, it's the fact that he's so confident about the future that is most relaxing.
Also, it's quite serene listening to him talk about gigantic space habbitats; it's such a good/calming thought that all humanity would be able to enjoy space, have all their basic needs met, saving Earth by offloading our resources exploitation elsewehere; I guess it's an engineer's wet dream :)
For me the sleep prep is more about momentum than duration, if I'm moving in the relaxing direction the duration of the prep is not as important and I can go to sleep shortly after watching TV or using my PC.