A period of sleep is known to benefit performance in memory tasks, but a study on page 122 of this issue suggests that it is not just the amount, but the kind, of sleep that is important.

This study recorded electroencephalograms from people as they slept and set off a beeping sound when the electroencephalograms were consistent with a sleep stage known as slow-wave sleep. Slow-wave sleep is a state of deeper sleep, so although the beep did not awaken the subjects, they slid out of slow-wave sleep into a different, shallower sleep stage.

Although the total amount of sleep that subjects got was unchanged, these people did worse on a later test of scene recall than subjects who had slept normally. Moreover, when the subjects were later scanned in a functional magnetic resonance imaging scanner, they also showed reduced hippocampal activation while they were encoding the to-be-remembered scenes. These results suggest that hippocampus-dependent memory is particularly affected by shallow sleep.