Humans during the sleep can strengthen previously acquired memories, but
whether they can acquire entirely new information remains unknown. The
nonverbal nature of the olfactory sniff response, in which pleasant
odors drive stronger sniffs and unpleasant odors drive weaker sniffs,
allowed us to test learning in humans during sleep. Using
partial-reinforcement trace conditioning, we paired pleasant and
unpleasant odors with different tones during sleep and then measured the
sniff response to tones alone during the same nights' sleep and during
ensuing wake. We found that sleeping subjects learned novel associations
between tones and odors such that they then sniffed in response to
tones alone. Moreover, these newly learned tone-induced sniffs differed
according to the odor pleasantness that was previously associated with
the tone during sleep. This acquired behavior persisted throughout the
night and into ensuing wake, without later awareness of the learning
process. Thus, humans learned new information during sleep.