For most of the twentieth century, sleep was regarded as a passive state, a pause in which the brain simply rested. Research over the past three decades has overturned that view. During sleep, and particularly during the deep, slow-wave stages that dominate the first half of the night, the brain appears to replay patterns of activity recorded during the day. In experiments with rats, neurons in the hippocampus that fired as an animal explored a maze fired again, in the same order, while it slept.
This replay seems to matter for learning. Volunteers who learned a list of word pairs in the evening and slept before being tested recalled more of them than volunteers who learned the same list in the morning and were tested after a day awake, even though the interval was the same length. The advantage was largest for material that had been only partly learned, which suggests that sleep does not merely protect memories from interference but actively strengthens the weaker ones.
None of this means that a student can replace study with sleep. Sleep consolidates what has been encoded; it cannot create memories of material that was never learned. Nor are the findings uniform: the benefit is clearer for facts and word lists than for complex skills, and some studies have failed to replicate it at all. Still, the evidence is strong enough to cast doubt on a habit common before examinations. A night spent revising instead of sleeping may add new information at the cost of the information already acquired.
