For the research, a group of lab rats were studied. First, the rats were conditioned to associate a specific odour with a mild electric shock while they were awake.
After the rodents learnt to make the association, demonstrated by enhanced fearful behaviour, the odour experience was re-created when the rats were in slow-wave sleep, a phase of deep sleep. When the rats during this phase of sleep received a replay of the odour experience they had learnt while they were awake, they demonstrated an increased memory of the odour in comparison with those in another test group that received a similar replay of the odour while awake.
However, when the dozing rodents received new odour information as they slept, they demonstrated greater difficulty in distinguishing the learnt odour they were previously exposed to while awake, compared to others. The researchers claim that this—the difficulty of telling a new odour apart from a learnt odour when asleep—is the reason that brains shut out sensory information while sleeping.
Donald Wilson, one of the researchers on the project, says, “We know that during slow-wave sleep, the brain’s sensory systems are far less responsive to normal inputs… Our data suggest this sensory isolation may help allow replay of learned information in the absence of external interference, providing strong, precise memory of important information.” One occasionally needs to switch off to retain what’s relevant.