Context: 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. Because normal people cannot entrain to a 28-hour day in dim light if at all,[citation needed] this is referred to as a forced desynchrony protocol. Sleep and wake episodes are uncoupled from the endogenous circadian period of about 24.18 hours and researchers are allowed to assess the effects of circadian phase on aspects of sleep and wakefulness including sleep latency and other functions.[page needed]

Question: What time cycle did studies in 1938 and 1990s use on humans?
Answer: 28-hour

Question: What conditions were suppressed in the 28 hour wake-sleep cycle studies ?
Answer: time cues

Question: How long did the suppression of time clues study last?
Answer: month

Question: What is this forced type of study called?
Answer: forced desynchrony

Question: When in the cycle do wake-sleep cycles break off from the circadian period?
Answer: 24.18 hours

Question: What did Kleitman do in the 1990's?
Answer: Unanswerable

Question: Whar dir Czeisler do in 1938?
Answer: Unanswerable

Question: When can normal people adjust to a 28 hr sleep cycle?
Answer: Unanswerable

Question: What happens at 24.5 hours?
Answer: Unanswerable