The two procedures interact on a permanent basis, let us remind ourselves. For example, at 19.00, sleep pressure is already heightened and in itself could push people to go to sleep. But the wake promoting circadian signal encourages people to stay awake, in a way counteracting the activity of the homeostatic mechanisms. Also, the cognitive performance tests carried out during a normal day did not allow the exact influence of the two components studied to be singled out. Nevertheless, after being awake for 40 hours, sleep pressure reaches a form of saturation. In these conditions, if it was observed that the performance of the subjects improved at certain moments of the day despite an ever increasing need to sleep, here the imprint of a circadian peak can be seen.

That is what the experiment carried out in Basel emphasized in an unequivocal manner: there do indeed exist circadian variations in performing the procedural task demanded. The researchers did not observe a linear decline in the performances according to sleep pressure; on the contrary modulations connected to variations in circadian wakefulness, including body temperature, were observed. In other words the performances were better during the ‘right circadian phase’, no matter the sleep pressure at work. Similar results were obtained in the second experimental procedure, where the sleep pressure was monitored in a different manner, through a series of siestas prescribed to the experiments’ participants.

Towards 6 o’ clock in the morning, body temperature is minimal and the secretion of melatonin maximal. In laboratory controlled conditions it is at this moment that performances concerning simple cognitive tasks are the worst for subjects whose chronotype could be called neutral. They then improve over the course of the day in parallel with the rise of body temperature, whose maximum measurement is reached around 22.00. It remains to be discovered what exactly happens in daily life, in which several factors play a part: the levels of the day’s light, the motivations, the journeys and the gestures carried out by the subject, etc.

The paths of extremes

For more complex tasks the data remains vague, as the results of the experiments revealed a lot of variability. Should one call into question the methodological reasoning? Maybe. Even so, an experiment carried out by Christian Cajochen and his University of Basel team seems to have underlined a pattern of performances analogous to that which had been established within the tests looking into the aforementioned procedural task. The second experiment followed the broad lines of the first one – a strictly controlled environment, sleep deprivation, etc. - but only looked at elderly people. The latter were asked to direct themselves through ‘paper-pencil’ versions of labyrinths. It appeared that the harder the task was, the more marked were the performances’ sensitivity to circadian influence. The work carried out at the University of Basel called on so-called ‘neutral’ subjects in terms of chronotype. Otherwise, in the perspective that they had fixed themselves, it would have been almost impossible to disentangle the web.