A siesta is a short nap taken in the early afternoon, often after the midday meal. Such a period of sleep is a common tradition in some countries, particularly those where the weather is warm. The word siesta is Spanish, from the Latin hora sexta - "the sixth hour" (counting from dawn, therefore noon, hence "midday rest").
The siesta is the traditional daytime sleep of Spain, and through Spanish influence, of many Latin American countries. Brazil, colonized not by Spain but by Portugal, stands in cultural contrast.
Factors explaining the geographical distribution are mainly high temperatures and heavy to very heavy intake of food at the midday meal. These two factors combined contribute to the feeling of post-lunch drowsiness. Afternoon sleep is also a common habit in countries along the equator. In these countries, the heat can be unbearable in the early afternoon, making a midday break at home ideal. In many areas with this habit, it is common to have the largest meal of the day in the very early afternoon, as is practical and common in farming.
The original concept of a siesta seems to have been merely that of a midday break intended to allow people to spend time with their friends and family. It has been suggested that the long length of the modern siesta dates back to the Spanish Civil War, when poverty resulted in many Spaniards working multiple jobs at irregular hours, pushing back meals to later in the afternoon and evening. However, this hypothesis sounds unlikely, considering that the siesta tradition was very common in Latin America and other countries with Hispanic influence, much before the Spanish Civil War.
Older, pre-teenage children are usually incapable of napping, but acquire the ability to nap as teenagers.
The timing of sleep in humans depends upon a balance between homeostatic sleep propensity, the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode, and circadian rhythms which determine the ideal timing of a correctly structured and restorative sleep episode. The homeostatic pressure to sleep starts growing upon awakening. The circadian signal for wakefulness starts building in the (late) afternoon. As Harvard professor of sleep medicine Charles A. Czeisler claims, "The circadian system is set up in a beautiful way to override the homeostatic drive for sleep."
Thus, in many people, there is a dip when the drive for sleep has been building for hours and the drive for wakefulness has not yet started. This is, again quoting Czeisler, "a great time for a nap."The drive for wakefulness intensifies through the evening, making it difficult to get to sleep 2–3 hours before one's usual bedtime when the wake maintenance zone ends.
In some individuals, postprandial dip, a brief drop in blood glucose levels caused by the body's normal insulin response to a heavy meal, may produce drowsiness after the meal that can encourage a nap. However, the appearance of the dip is primarily circadian as it occurs also in the absence of the meal.