Napping and memory for word pairs

Introduction: One theory of why we sleep is that sleep helps the brain consolidate, or stabilize, newly learned information. In this study, researchers tested whether a short daytime nap protects newly learned word pairs from being forgotten.

Participants: Sixty students (34 women and 26 men) aged 18 to 22 at one public university in a large city responded to campus flyers and were paid $40. They reported their race/ethnicity as 35% White, 25% Hispanic or Latino, 20% Asian, 15% Black and 5% multiracial. People who reported a diagnosed sleep disorder, or who worked night shifts, were not eligible.

Method: Each participant gave written informed consent and was told they could stop at any time. Data were stored under ID numbers rather than names. At 1:00 p.m., each participant studied 40 pairs of unrelated words (such as candle–river) on a computer, repeating the list until they could correctly type the second word of at least 24 pairs when shown the first word. Each participant was then randomly assigned to one of two groups for the next 90 minutes.

Nap group (30 participants): Participants slept in a dark, quiet lab bedroom. Electrodes on the scalp recorded EEG brain-wave patterns to identify sleep stages.

Wake group (30 participants): Participants sat in a dimly lit lab room watching a nature documentary, with a researcher present to make sure they stayed awake.

All participants then rested quietly for 60 minutes so that anyone who had napped would not be groggy. At 4:00 p.m., everyone was shown the first word of each of the 40 pairs and asked to type the second word. Afterward, participants were told the purpose of the study and the hypothesis being tested.

Results and Discussion: The table shows the results. On average, the nap group spent 21 minutes in NREM stage 3 (deep, slow-wave) sleep. The difference between the two groups' means at the 4:00 p.m. test was statistically significant (p < .01). Within the nap group, participants who spent more minutes in stage 3 sleep tended to recall more pairs at the final test (r = +0.46). The researchers concluded that the nap helped protect the new memories from being forgotten.