Time-of-Day Variation in Task Performance
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.
One of the major influences on time-of-day variations in physiology and behaviour is the activity of internal rhythm generating systems. Circadian (about 24 h) rhythms, are controlled by a master biological clock. In mammals, the master biological clock is located in the suprachiasmatic nuclei of the hypothalamus.(12) At the subcellular level of organisation, circadian rhythms are generated by transcriptional and translational feedback loops involving multiple clock genes.(13) The precise periodicity (or cycle length) of the biological clock is known to be genetically determined,(14) and variation in clock genes is thought to be related to individual differences in natural wake and sleep times.(15)
The biological clock generates and maintains circadian rhythms in most physiological, biochemical, and behavioural variables–for example, core body temperature, triacylglycerol, blood pressure, sleep-wakefulness, alertness, mental performance, and the synthesis and secretion of many hormones including melatonin, cortisol, prolactin, and growth hormone (some of these are shown in figure 1). A reliable and extensively researched marker of biological-clock activity is the rhythm of melatonin. Melatonin is the principal hormone of the pineal gland. It is synthesised and secreted at night in both day-active and night-active species, thereby acting as a signal for the length of day and night. In human beings, sleep is normally initiated during the rising phase of the melatonin rhythm and declining phase of the body temperature rhythm. Attempts to sleep at inappropriate phases of the circadian cycle, for example during the declining phase of melatonin and rising phase of body temperature, will usually result in shorter sleep episodes and more awakenings.(16) Such attempts are frequent in workers on night shifts.
Light is the major synchronising agent for mammalian circadian rhythms. Results of studies have shown that exposure to even low light levels (100 lux), similar to that found in offices and living rooms, will substantially affect the phase of human circadian rhythms.(17) However, without scheduled activities and sleep, such intensities seem incapable of maintaining optimum synchronisation to the 24-h day.
Responses to light depend on the time of exposure in relation to the internal biological clock: exposure to light just after the body temperature minimum will advance the phase of circadian rhythms, whereas exposure before the body temperature minimum will induce delays.(18) Core body temperature is usually at a minimum around 0400-0600 h, but it can be substantially displaced by shiftwork, jet-lag, and other situations. Time of day-dependent responses are usually described according to a phase response curve (PRC; figure 2). PRCs can be used to predict the timing of light treatment to enable adaption to environmental changes, such as those seen in shift-work and transmeridian travel.
In continuous darkness or in dim domestic intensity light and in the absence of other important time cues such as an imposed sleep-work schedule, human rhythms free run, or become desynchronised from the 24-h day and express the underlying periodicity of the biological clock. This is often seen in blind people who have no conscious light perception.(19) Rhythms can be synchronised by weak time cues, but have an abnormal phase relation with the environment.(20) An example is the tendency to oversleep in winter (dim light), which in polar regions (especially in individuals with no behavioural impositions such as scheduled sleep wakefulness and work times) can become an overt free run.(21) For those working indoors during a normal day (0800-1700 h), bright natural early morning light is only seen in the summer in the higher latitudes of temperate or polar regions, and this early morning light exposure might well result in earlier circadian phase.
Timed exercise can also shift the human biological clock, however, to date mainly phase delays have been shown.(22) Appropriately timed administration of melatonin can, in addition to inducing sleepiness, phase shift and synchronise the human circadian system.(23,24) In countries where melatonin is freely available, it is extensively, indiscriminately, and no doubt often inappropriately, used as a treatment for circadian rhythm disorders and as a sleeping pill.