Slow wave sleep is the deepest level of sleep, but its function is not fully understood. William Fishbein, at the City University of New York, suspected that it might play a role in memory formation, even when the sleeper reaches this level of sleep during a short nap. Researchers taught English-speaking students lists of Chinese words such as mother, sister, and maid, which are spelled with two characters in Chinese. They then had half of the subjects take a 90-minute nap and the other half remain awake. They made sure that the sleeping students stayed in slow wave sleep during their naps.
...[I]t appears to be not the length of the sleep that matters so much, but rather the quality.