I use online chess tactics problems to assess how good my brain is that day. I estimate there's a 300+ ELO variance, perhaps even more, in my performance between when I'm tired and when I'm rested.

300 ELO points between 2 players means that the higher rates one wins about 9 out of 10 games against the lower rates one. A huge difference in performance.

First thing I do at my computer if I am groggy is to do chess tactics. Based on the results, I take a break or continue with my actual work. I should probably be looking at ELO as well, I just go by numbers at this time !

Sleep seems to be very cyclical and internalized in the body, so it isn't surprising that a drastic change would have such a dramatic effect. I wonder if anyone could gain the ability to only need 6 hours of sleep by very slowly decreasing sleep over a long time (say 6-12 months). But your body will probably never adapt to this 6 hours on the weekdays, 10 hours on the weekends stuff that seems to be common.

The people I know who sleep 5-6 hours a night consistently have been doing it for a very long time and describe the feeling of sleeping 8 hours the same way I would describe sleeping 13 hours. Just feeling slow and almost like being sleep deprived all day.

Speaking a foreign language is that meter for me. If I don't get proper sleep, my tongue doesn't do what I want it to. I slip up more in my pronunciation and I forget words that I need. More sleep has always equalled higher fluency for me.

> I used to habitually play bullet and blitz chess online, which confirmed the necessity of sleep for me

60-minute games made the same apparent to me. Instantaneous drops and rises, solely resulting from sleep (or the lack thereof).