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Students preparing for examinations often sacrifice sleep in order to study for longer, but research on memory suggests that this strategy may be self-defeating. Learning does not end when a person closes a book. A newly formed memory is fragile, and over the following hours it must be stabilised and connected to existing knowledge, a process psychologists call consolidation. A large body of evidence indicates that sleep plays an active part in this process. In typical experiments, volunteers learn a list of word pairs or a new skill and are tested some hours later; those who have slept in the interval usually remember more than those who have stayed awake for the same length of time. Recordings of brain activity help to explain why. During deep sleep, patterns of activity that appeared while a person was learning are replayed, as if the brain were rehearsing the day's experiences, and information appears to be gradually transferred to longer-lasting networks in the brain. Sleep also matters before learning. People who have been deprived of sleep find it harder to take in new information the next day, partly because the brain regions that form new memories work less efficiently when tired. None of this means that sleep can replace study; material that was never learned cannot be consolidated. However, it does suggest that a regular pattern of adequate sleep is part of effective learning rather than time taken away from it, and that staying up all night before an exam may cost more than it gains.