In a statement published on its website, the Federal Penitentiary Service said that Navalny "felt unwell" after a walk on Friday, and "almost immediately lost consciousness."
"The conditions violate human dignity," the report stated.
Prisoners are afforded nowhere near the four-and-half square meter minimum afforded them under international law.
Imran Khan had been sentenced to 10 years for leaking state secrets and 14 years along with his wife for illegally selling state gifts.
In a message on X facilitated by his allies, Navalny described a surreal morning routine.
Convicted rapist Josef Fritzl may soon be released from prison as his defense attorney argued Fritz’s deteriorating health means he no longer poses a public threat.
Navalny, who has been sentenced to stay in jail until he is 74, has repeatedly warned that Putin's Russia is a state run by "thieves and criminals" and that one day there will be a revolt.