Sleep is usually treated as time subtracted from the day, yet the hours spent asleep are far from empty ones for the brain. Through the night it repeats the patterns of activity produced while a lesson or a practical skill was originally being learned, and the material that matters is gradually moved into longer-lasting storage. Students who sleep properly after an evening of study generally recall considerably more of it than classmates who carry on working into the early hours. A complete night of sleep before an examination is therefore usually worth more than the additional revision that replaces it.