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Air pollution might be linked to poor sleep, say researchers looking into the impact of toxic air on our slumbers. The study explored the proportion of time participants spent asleep in bed at night compared with being awake - a measure known as sleep efficiency. The results reveal that greater exposure to nitrogen…

• 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?