The inner mechanics of the biological clock turns out to be very complex, but seem today to have been relatively well explained for the small fruit fly and the mouse. In humans the period of the biological clock is slightly higher than 24 hours, which brings into play the notion of circadian rhythms. Furthermore, our biological clock is kept in step with our environment by elements of our physical and social environment which are called ‘sleep synchronisers’, at least in ‘normal’ conditions. The main synchroniser is made up by variations in brightness brought about by alternating between day and night, but there are others such as physical activity and having meals.

Morning or evening, lark or owl?

It is in this context that the researchers at the University of Liège’s Cyclotron Research Centre (CRC) have taken an interest in human cognitive performances. We know not only that the latter fluctuate depending on the particular moment of the day but also that individuals are not affected in an identical manner by this phenomenon.

Circadian rhythms and sleep pressure (or the homeostatic process, as it is also called) results from the number of hours a subject has been awake and permanently interacting. For example, at 19.00 the sleep pressure is already high and could by itself provoke sleep. But the circadian signal for its part still causes wakefulness, in a certain way counterbalancing the activity of the homeostatic processes. Then, later in the evening, it begins to diminish and, having become too weak to counter sleep pressure, opens the door to sleep mode.

We nevertheless have to take into account inter-individual differences which modulate this interaction between circadian rhythm and sleep pressure. In effect there exist different ‘chronotypes.’ Certain individuals do not have a marked time preference: they are neither ‘morning’ nor ‘evening.’ Others like to rise early, perform well in the morning but go to bed at an early hour. Conversely, others find it difficult to get up early in the morning, prefer to work in the evening and go to bed in the middle of the night;

Significant differences

An FNRS Research Fellow, Christina Schmidt was the driving force behind a study, published in Science (1) in 2009, which brought to light ‘morning extremes’ and ‘evening extremes’ (see the article Are you an evening or a morning person?). A visual attention test was offered to a group of various people. The results? Some were more resistant to sleep deprivation than others. At the same time the suprachiasmatic area and the locus coeruleus, two regions which are strongly interconnected and highly involved in the circadian signal which underlies the state of wakefulness and regulates our level of vigilance when awake, were activated differently in the two groups.

(1) Schmidt, C., Collette, F.,  Leclercq, Y., Sterpenich, V., Vandewalle, G.,  Berthomier, P., Berthomier, C.,  Philipps, C., Tinguely, G., Darsaud, A., Gais, S.,  Schabus, M.,  Desseilles, M., DangVu, T., Salmon, E., Balteau, E., Degueldre, C., Luxen, A., Maquet, P., Cajochen, C., & Peigneux, P. (2009). Homeostatic Sleep Pressure and Responses to Sustained Attention in the Suprachiasmatic Area, dans Science 324, 516.