Reading Comprehension

Read the passage and answer the question.

Sleep is not a single uniform state but a cycle of distinct stages that repeat through the night. In the deepest stage, the body repairs tissues and strengthens the immune system. In another stage, marked by rapid eye movement, the brain becomes highly active and most vivid dreaming occurs; this phase appears important for sorting and storing memories. A full cycle lasts roughly ninety minutes, and a healthy sleeper passes through several each night. Waking up in the middle of a deep stage often leaves a person groggy, while waking at a cycle's end feels easier.