Sleep is a state of rest, which happens in animals, including humans. Animals that sleep are in an unconscious state, or mostly so. Most of the muscles that animals can control on purpose are inactive. Animals that are asleep do not react to stimuli as fast as those that are awake. They can wake up from sleep more easily than from hibernation or coma. All mammals and birds, and many reptiles, amphibians, and fish, sleep. In humans, other mammals, and most other animals that were studied, sleeping regularly is essential for survival.