Two processes

Most research work related to variations in cognitive performance over the course of the day has focused on the influence of the homeostatic process, expressing the need to sleep, and the circadian process which, in the words of Christina Schmidt, FNRS candidate at the University of Liège, constitutes a clock of the propensity to sleep and modulates the thresholds of wakefulness over the course of the day.

The ‘game’ the two processes become engaged in is not an innocent one, because it determines the daily variations of fatigability or overall attentiveness, and by the same token can affect performance levels over a series of cognitive tasks: arithmetical, reaction times, short term memory, etc. (Read the multiple memory). Studies carried out up until now have enabled a separating out of the influence of these two components, homeostatic and circadian, but have usually only looked at tasks involving relatively simple cognitive processes, such as speed and vigilance tests.

In these conditions the performances attained by the subjects coincide with the circadian body temperature curve, which constitutes a faithful reflection of the state of wakefulness over the course of the nycthemeron. In the absence of well established data we nevertheless do not know if more evolved tasks, relying for example on memory or executive functions, are housed at the same lodgings.

Several studies have looked into the links between the chronotype and cognitive performance. They reveal that the latter is at its best or, conversely, at its worst, at a precise moment during the day, itself a function of the subject’s neutral, early rising or evening profile. As Christina Schmidt indicates, this chronotypical susceptibility has been observed for tasks exploring various aspects of our executive functions, but seems to affect the production of routine or conditioned responses less, or even not at all.

Swiss experiments

Beyond the homeostatic process, circadian rhythm and the chronotype, a fourth factor needs to be taken into account: age. De facto, over the course of ageing, human beings tend to become more and more ‘morning people.’ ‘The interaction between cognitive variables and the moment of the day changes over the course of ageing, young and old subjects differing significantly in their optimal circadian peak,’ states Christina Schmidt.

Very interesting experiments have been carried out at the University of Basel’s Centre for Chronobiology, led by Professor Christian Cajochen, a laboratory in which Christna Schmidt carried out a one and a half year training programme (2005-06). Her work was carried out in a very controlled environment, and one which was very restrictive for the volunteers who took part in them. During a full week each of them had to stay in a room in which the eating and lighting conditions, as well as the body posture to be assumed (half lying down), were rigorously defined.

The volunteers were offered a so called procedural task, in this case a motor skills test. They were asked to place themselves in front of a computer screen on which had been fixed dots below which a visual clue could appear. A button corresponded to each of the dots. The task constituted in pushing the right one as quickly as possible when the visual prompt appeared on the screen.

Every volunteer was submitted to two experimental procedures. In the first they were deprived of sleep for 40 consecutive hours; as for the second, it hinged on half an hour siestas every 90 minutes. For the first experiment the goal was to increase sleep pressure beyond a threshold which allowed for the individual impact of circadian and homeostatic mechanisms on cognitive performance to be singled out.