A daytime nap restores hippocampal function and improves declarative learning.

STUDY OBJECTIVES: Daytime naps can confer benefits on subsequent declarative learning, but the physiological correlates of this improvement are less well studied. We examined learning following a daytime nap compared to an equivalent waking period using fMRI and polysomnography (PSG). METHODS: Forty healthy young adults who slept normally the previous night encoded word pair lists in an MRI scanner at 1PM and 4.30PM. Between sessions, participants either stayed awake and watched a documentary (Wake Group; N=20) or had a 90-min nap opportunity (Nap Group; N=20) monitored by PSG. Approximately 40-min after completing each encoding session, memory for learned words was assessed using cued-recall. RESULTS: A significant Session x Group interaction effect (p<0.001) was observed in which memory was significantly improved in the Nap but not in the Wake group (p<0.001). There was also a Session x Run x Group interaction effect in the left hippocampus (p=0.001), whereby activation during word-pair encoding increased only following the nap. Both performance improvement (rs=0.46, p=0.04) and nap-related increase in hippocampal activation (rs=0.46, p=0.04) were correlated with nap spindle count (12-15 Hz) but not with slow oscillation power (p=0.18). CONCLUSIONS: After a habitual nocturnal sleep, participants who had a 90-min afternoon nap encoded word pairs better than a comparable group who stayed awake. Increases in hippocampal activation following the nap suggest restored hippocampal function. Naptime spindles may contribute to improved memory.