This task is about reading the given passage and construct a question about the information present in the passage. Construct a question in such a way that (i) it is unambiguous, (ii) it is answerable from the passage, (iii) its answer is unique (iv) its answer is a continuous text span from the paragraph. Avoid creating questions that (i) can be answered correctly without actually understanding the paragraph and (ii) uses same words or phrases given in the passage.
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Question: Studies by Nathaniel Kleitman in 1938 and by Derk-Jan Dijk and Charles Czeisler in the 1990s put human subjects on enforced 28-hour sleepwake 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]

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

Question: After the lengthy Iraq disarmament crisis culminated with an American demand that Iraqi President Saddam Hussein leave Iraq, which was refused, a coalition led by the United States and the United Kingdom fought the Iraqi army in the 2003 invasion of Iraq. Approximately 250,000 United States troops, with support from 45,000 British, 2,000 Australian and 200 Polish combat forces, entered Iraq primarily through their staging area in Kuwait. (Turkey had refused to permit its territory to be used for an invasion from the north.) Coalition forces also supported Iraqi Kurdish militia, estimated to number upwards of 50,000. After approximately three weeks of fighting, Hussein and the Ba'ath Party were forcibly removed, followed by 9 years of military presence by the United States and the coalition fighting alongside the newly elected Iraqi government against various insurgent groups.

Answer: What demand did the US make before invading Iraq in 2003?

Question: French historians traditionally place the Enlightenment between 1715, the year that Louis XIV died, and 1789, the beginning of the French Revolution. Some recent historians begin the period in the 1620s, with the start of the scientific revolution. The Philosophes, the French term for the philosophers of the period, widely circulated their ideas through meetings at scientific academies, Masonic lodges, literary salons and coffee houses, and through printed books and pamphlets. The ideas of the Enlightenment undermined the authority of the monarchy and the church, and paved the way for the revolutions of the 18th and 19th centuries. A variety of 19th-century movements, including liberalism and neo-classicism, trace their intellectual heritage back to the Enlightenment.

Answer:
What year did the Enlightenment begin according to French historians?