If they work long nights, they will be dumb eventually. Permanent stupidity can be induced by a relatively a short period of sleep deprivation.

Umm, proof?

I don't have that, I should probably have been clearer that this was my own observation and view, but I felt that there was something that happened when a particular highschooler failsed to sleep and stayed up using caffeine pills and I feel that it made the person more rigid, less creative, something like that; and then I saw others who were on the other end of the spectrum, whose parents made sure that they slept, and they were just smarter.

I have no data, no scientific studies. It's just that I don't see how it can be any other way.

Alternatively, maybe caffeine and other stimulants decrease creativity and lead to what I felt became stupidity.