People really underestimate sleep on these forums. Really sleep is durned amazing, being able to sleep is like having some sort of a superpower, doesn′t make sense how immensly good for your body and mentality sleeping is.
If you get a good nights sleep then a lot happens, to mention something that happens to the body for starters, as you sleep the body reconstructs bad and damaged cells, it removes those it does not need, repairs those that are damaged and creates new cells where needed. And when it comes to the mind and learning, the brain processes most of the information you come across over the day and engraves it into your long term memory, you can even perform better at a task the day after sleeping than while learning the task the day before.
I can take a very recent example, but self stories are boring, so feel free to skip my little story of self, it′s here though if you′re intrested.
Spoiler I was trying to learn how to count to ten in Japanese the other day when I kept having problems when recounting the numbers outloud, I′d either forget one number, switch them or accidentally say a different number than I intended (this can happen for example with the number 4, I was trying to learn the prefered way of counting where you say yon but no shi, but because the number 3 is 'san' I kept saying shi by accident just out of habit since I first read the numbers that way and it rolls from your toung a lot easier saying "san shi" than "san yon") so basically while it′s a task that is pretty easy to do I was pretty tired and couldn′t get it right after 8 tries in a row... (D: yeah I′m slow...) so I decided to just go to bed and try the next day since I was pretty darn tired.
The next day I had forgotten all about that I was going to go over the numbers again and I went on my way to prepare for work, not having google or any word book around I decided to try and go over the numbers by memory, only to find that I could recount the exact row of the numbers perfectly on the first try, I hadn′t even expected to manage that without checking the numbers first so I went over the row in my mind a few more times and then checked on the internet... and I was getting it right. Your mind is a lot clearer after a good nights sleep and you process the information you were collecting over the day really well during sleep. Now of course it doesn′t matter if I′m tired or not, it′s something I just remember as clearly as I remember how to count to ten in my home language and in English, it′s just too ingraved in my mind to forget by now.
So I don′t think there is any reason why sleep would be so bad in Nirn that people could get by without it, the warriors just pump their muscles and don′t need any sleep to let the body heal any ruptured capillaries and the mages don′t need to memorize things for the long term by sleeping, their mind is always clear and they learn things 100% on their first try, pff who needs REM sleep anyway for the consolidation of memories, not great Nirn mages, nu-uh.