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.
In the second part of the experiment (post-nap for the nappers), the subjects were given a list of Chinese words that they had not been exposed to earlier. The napping group performed much better on the test, clearly having made the connection that the first character on the earlier list of words was constant (in the above example, that the first character always meant "female." Their chances were also greater of guessing that a new word containing the female character might mean "princess" rather than one with no such gender identity.
"The nap group has essentially teased out what's going on," said Fishbein. In other words, the nappers pulled out the meaning of the first character in the list of words, and this association seems to have been made during slow wave sleep. This suggests that the brain may use slow wave sleep to solidify relational memory, and that it appears to be not the length of the sleep that matters so much, but rather the quality.
As Fishbein concludes, "not only do we need to remember to sleep, but most certainly we sleep to remember." The moral of the story? Don't feel bad about taking a nap — you're helping your brain do valuable work, even if you're not conscious for it.