Afternoon is the time of day from noon or lunchtime until evening. There is no exact definition of the time when afternoon ends and evening starts. (This is equally true for "evening" and "night".) However afternoon is often taken to start at midday (12:00 noon). Afternoon ends and evening starts at roughly 6 pm or maybe just before sunset. Afternoon is the time when the Sun is descending from its zenith in the sky to somewhat before its terminus in the westerly horizon. In human life, it occupies roughly the latter half of the standard working and school day.
Afternoon is often defined as the time period between noon and evening. If this definition is adopted, the specific range of time varies in one direction: noon is at 12:00 (for simplicity, using the 24 clock ("military time")), but the boundary between afternoon and evening has no standard definition. The time period of evening is subjective, but it is often socially considered to start around 6 pm and shortly before sunset, during twilight (these may be conflicting, since the time of sunset (and twilight) vary a lot depending on season and latitude, as well as the choice of time zone)..
Before a period of transition from the 12th to 14th centuries, "noon" instead referred to what we now call 3 pm. Possible explanations include shifting times for prayers and midday meals, along which one concept of "noon" was defined—and so afternoon would have referred to a narrower timeframe.
The word "afternoon", which derives from "after" and "noon", has been attested from about the year 1300; Middle English contained both "afternoon" and the synonym "aftermete". The standard phrasing was "at afternoon" in the 15th and 16th centuries, but has shifted to "in the afternoon" since then. The term should not be confused with "after noon" (two separate words), which is a translation of the Latin post meridiem (p.m.), meaning a time between 12:00 midday and 12:00 midnight.
Afternoon is a time when the sun is descending from its daytime peak. During the afternoon, the sun moves from roughly the center of the sky to deep in the west. In late afternoon, sunlight is particularly bright and glaring, because the sun is at a low angle in the sky. The standard working time in most industrialized countries goes from the morning to the late afternoon or evening—archetypally, 9 am to 5 pm—so the latter part of this time takes place in the afternoon. Schools usually let out during the afternoon as well.
Effects on living organisms
Hormones and body temperature
In diurnal animals, it is typical for blood levels of the hormone cortisol—which is used to increase blood sugar and aid metabolism and is also produced in response to stress—to be most stable in the afternoon after decreasing throughout the morning. However, cortisol levels are also the most reactive to environmental changes unrelated to sleep and daylight during the afternoon. As a result, this time of day is considered optimal for researchers studying stress and hormone levels. Plants generally have their highest photosynthetic levels of the day at noon and in the early afternoon, owing to the sun's high angle in the sky. The large proliferation of maize crops across Earth has caused tiny, harmless fluctuations in the normal pattern of atmospheric carbon dioxide levels, since these crops photosynthesize large amounts of carbon dioxide during these times and this process sharply drops down during the late afternoon and evening.
In humans, body temperature is typically highest during the mid to late afternoon. However, human athletes being tested for physical vigor on exercise machines showed no statistically significant difference after lunch. Owners of factory farms are advised to use buildings with an east–west (as opposed to north–south) orientation to house their livestock, because an east–west orientation generally means thicker walls on the east and west to accommodate the sun's acute angle and intense glare during late afternoon. When these animals are too hot, they are more likely to become belligerent and unproductive.
The afternoon, especially the early afternoon, is associated with a dip in a variety of areas of human cognitive and productive functioning. Notably, motor vehicle accidents occur more frequently in the early afternoon, when drivers presumably have recently finished lunch. A study of motor accidents in Sweden between 1987 and 1991 found that the time around 5 pm had by far the most accidents: around 1,600 at 5 pm compared to around 1,000 each at 4 pm and 6 pm. This trend may have been influenced by the afternoon rush hour, but the morning rush hour showed a much smaller increase. In Finland, accidents in the agriculture industry are most common in the afternoon, specifically Monday afternoons in September.
One psychology professor studying circadian rhythms found that his students performed somewhat worse on exams in the afternoon than in the morning, but even worse in the evening. Neither of these differences, however, was statistically significant. Four studies carried out in 1997 found that subjects who were given tests on differentiating traffic signs had longer reaction times when tested at 3:00 pm and 6:00 pm than at 9:00 am and 12:00 pm. These trends held across all four studies and for both complex and abstract questions. However, one UK based researcher failed to find any difference in exam performance on over 300,000 A-level exam papers sat in either the morning or afternoon. 
Human productivity routinely decreases in the afternoon. Power plants have shown significant reductions in productivity in the afternoon compared to the morning, the largest differences occurring on Saturdays and the smallest on Mondays. One 1950s study covering two female factory workers for six months found that their productivity was 13 percent lower in the afternoon, the least productive time being their last hour at work. It was summarized that the differences came from personal breaks and unproductive activities at the workplace. Another, larger study found that afternoon declines in productivity were greater during longer work shifts.
It is important to note, however, that not all humans share identical circadian rhythms. One study across Italy and Spain had students fill out a questionnaire, then ranked them on a "morningness–eveningness" scale. The results were a fairly standard bell curve. Levels of alertness over the course of the day had a significant correlation with scores on the questionnaire. All categories of participants—evening types, morning types, and intermediate types—had high levels of alertness from roughly 2 pm to 8 pm, but outside this window their alertness levels corresponded to their scores.
- Oxford English Dictionary, Third Edition (online), entries at afternoon and evening.
- "Afternoon". Merriam-Webster. Retrieved October 9, 2014.
- "Noon". Merriam-Webster. Retrieved October 9, 2014.
- "noon (n.)". Online Etymology Dictionary. 2001. Retrieved October 10, 2014.
- "afternoon (n.)". Online Etymology Dictionary. 2001. Retrieved October 10, 2014.
- Aggarwal & Upadhyay 2013, p. 172
- "Nine-to-fiver". Merriam-Webster. Retrieved October 9, 2014.
- Blaskovich 2011, p. 74
- Sinclair & Weiss 2010, p. 118
- Refinetti 2006, p. 556
- McCabe 2004, p. 588
- Refinetti 2006, p. 559
- McCabe 2004, p. 471
- McCabe 2004, p. 590
- Quartel, Lara (2014). "The effect of the circadian rhythm of body temperature on A-level exam performance". Undergraduate Journal of Psychology. 27 (1).
- Ray 1960, p. 11
- Ray 1960, p. 12
- Ray 1960, p. 18
- Refinetti 2006, p. 561
- Aggarwal, Anjali; Upadhyay, Ramesh (2013). Heat Stress and Animal Productivity. Springer Publishing. ISBN 978-8-132-20879-2.
- Blaskovich, Jim (2011). Social Psychophysiology for Social and Personality Psychology. ISBN 978-0-85702-405-3.
- Ekirch, A. Roger (2006). At Day's Close: Night in Times Past. W. W. Norton & Company. ISBN 978-0-393-32901-8.
- McCabe, Paul T. (2004). Contemporary Ergonomics. CRC Press. ISBN 0-8493-2342-8.
- Ray, James T. (1960). Human Performance as a Function of the Work–Rest Cycle. National Academy of Sciences.
- Refinetti, Roberto (2006). Circadian Physiology (2nd ed.). Taylor & Francis Group. ISBN 978-0-8493-2233-4.
- Sinclair, Thomas M.; Weiss, Albert (2010). Principles of Ecology in Plant Production. University of Nebraska, Lincoln/University of Florida.