Some students think that staying up late before a test improves their results. But studies show the opposite. During sleep, the brain arranges the facts we learned in the day. It moves them from short-term memory to long-term memory. If a person sleeps only a few hours, his focus becomes weak, his reactions become slow and he forgets more. An adult usually needs seven or eight hours each night. A child needs more than that. For this reason, experts advise students to review lessons early and then get enough sleep. They should not depend on one tiring night. Sleep is a part of learning, not wasted time.

Question: How many hours does an adult usually need to sleep, according to the text?