Sleep is a state of resting, which happens in animals, including humans. During sleep, most of the muscles that animals can otherwise control are not active, re-gaining the energy for the next time they wake up. Animals during sleep are usually in an unconscious, relaxed state. Like most animals, healthy sleep in humans takes place at night. Asleep describes the condition where animals are in the process of sleeping. During this time, they will not react as quickly (if at all) as they would if they were awake. They can, however, wake up from sleep more easily than from hibernation or coma. All mammals, birds, many reptiles, amphibians, and fish, have a sleep cycle. In humans, other mammals, and most other animals that were studied, regular sleep is essential for survival.