Question: What conditions were suppressed in the 28 hour wake-sleep cycle studies ?
Sentence: Studies by Nathaniel Kleitman in 1938 and by Derk-Jan Dijk and Charles Czeisler in the 1990s put human subjects on enforced 28-hour sleep–wake cycles, in constant dim light and with other time cues suppressed, for over a month.
Answer in sentence: yes