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Air pollution may be linked to poor sleep, according to researchers studying the effects of toxic air on our rest. Their study looked at the proportion of time that participants spent asleep in bed at night, compared with the time they spent lying awake - a measure known as sleep efficiency. In other words, sleep…

• The writer defines sleep efficiency in the first paragraph as...
• What reference information does the writer tell us the researchers used as a basis for their study, in the second paragraph?