Sleep (also called napping) is a state of resting. It happens in most animals, including humans. To sleep is to be unconscious.
During deep sleep, most of the muscles that animals can control are not active, but resting to regain the energy they need for the next time they wake up. When asleep, humans cannot tell what is happening around them. Like most animals, healthy sleep in humans most often happens at night.
When something is asleep, it cannot react like it would if it was awake. However, waking up from sleep is easier than waking up from hibernation or a coma. All mammals and birds, as well as many reptiles, amphibians and fish have a sleep cycle. In humans and many other animals, regular sleep is essential for survival.
It seems that all animals with complex brains have sleep. We know that sleep is extremely important to human health and well-being. Humans and animals need sleep but we do not know exactly why. If one is tired (from not getting enough sleep), one will not be able to function properly in common activities. Being sleep-deprived leads to struggling to remember information, changing one's mood, energy, health, focus and a number of other effects. This is why people are told to not drive or use big machines when they feel tired. Sleep deprivation (not allowing a person enough sleep) can even be used as torture. Sleep deprivation causes negative effects for the person such as: irritability (annoyed mood), forgetting things or amnesia, and some pains (for example back ache or neck pain) in different areas of the body. Also, the immune system normally releases compounds known as cytokines which are used to help fight inflammation and infection. If a person does not get enough sleep, their body cannot protect them from illness. The body may not have time to complete memory recollection, muscle repair, and release hormones that regulate growth and appetite.
During daytime, the Sun is out, and most people are awake. People work, go to school, or complete daily errands and activities. Some people sleep for a short time in the early afternoon for a quick rest—or because they are not able to sleep during the day. This is called a nap. A good nap should is recommended to last between 15 and 30 minutes. Naps that are 30–60 minutes can make someone feel dazed, confused, and less focused. It can also make it harder to sleep at night.
In some countries, most notably where the weather is warm, there is a tradition to take a nap early in the afternoon. This tradition is called siesta. It is most common in Spain and Latin America. Some stores and services close while their owners and/or employees take their siesta. It is common because the day gets hottest when the sun is up in the sky and strong. Therefore, people do their activities early in the morning and later in the evening, when it is cooler.