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] COUNT 5
QSN1 What time cycle did studies in 1938 and 1990s use on humans? ANS1 28-hour QSN2 What conditions were suppressed in the 28 hour wake-sleep cycle studies ? ANS2 time cues QSN3 How long did the suppression of time clues study last? ANS3 month QSN4 What is this forced type of study called? ANS4 forced desynchrony QSN5 When in the cycle do wake-sleep cycles break off from the circadian period? ANS5 24.18 hours