More than a century ago, it was reported that the capacity for doing mental work varies throughout the day. Several empirical studies have revealed time of day variations in performance, with subtle differences between different tasks. Similarly, in participants that are exposed to 36-60 h of sustained wakefulness in controlled laboratory (or constant routine) conditions, significant time of day variations in task performance are reported, with performance being worst for all tasks just after the time of core body temperature minimum (about 0400-0600 h). Subjective alertness levels are closely related to the time-of-day variation in task performance.
In the UK, as in many other countries, sleep-related vehicle accidents peak in the second half of the night (0200-0600 h), and also show a very modest rise in the mid afternoon (1400-1600 h).(10) The modest rise in accidents in the mid afternoon (which is small compared with the nocturnal rise) could reflect the post-lunch decrement in performance.(8) When variation in traffic density is taken into account, the likelihood of a sleep-related vehicle accident is 20 times higher at 0600 h than at 1000 h. Similarly, the risk of injury and fatality during the night shift is significantly greater than it is during traditional daytime working hours.(4,11) The cause of such accidents and injuries is often multifaceted, and the precise contribution of sleepiness is difficult to estimate.