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 are 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 2:00 pm to 8:00 pm, but outside this window their alertness levels corresponded to their scores. See also Morning Sunset Twilight 12-hour clock Night owl (person) References Further reading External links Parts of a day