We are not all equal before sleep deprivation. After a sleepless night, subjects who have inherited allele 4/4 of the PERIOD3 ‘clock gene’ prove to be less sensitive to sleep deprivation than those who have the other form, allele 5/5. As The Journal of Neuroscience has recently related, researchers at the Cyclotron Research Centre have brought to light the different brain functions of these two populations.
Human beings are a diurnal species. Nevertheless certain of us are placed in the position of working during the night and, moreover, often according to variable hours. In other words, the socio-economic imperatives our societies are faced with sometimes impose a programme which is incompatible with our own genetics.
Circadian biological rhythms and sleep deprivation thus constitute fertile ground for research, all the more so as we are not all equal before sleepless nights, on the one hand, and shift work – for example the classic ‘3 X 8’ system with weekly rotations – on the other. Some people carry out their nightly professional tasks without the slightest inconvenience, and even make light work of changes in shifts, whilst others find it hard to maintain their performance levels in the same circumstances. Persistent fatigue, problems sleeping, irritability, problems of concentration and memory recall can be their lot.
There is in addition a circadian rhythm of vigilance and cognitive performance which makes some individuals who do not tolerate sleep deprivation very well candidates for ‘human error’ – but, in these circumstances, is the term really an appropriate one? ‘A number of recent industrial disasters have occurred in the second part of the night, when sleep pressure, in other words the need to sleep, is not countermanded by a sufficiently strong circadian vigilance signal,’ points out Professor Pierre Maquet, a FNRS research director. And he cites several evocative names Three Mile Island, Bhopal, Seveso, and Chernobyl.
In effect, all living organisms possess an internal clock which allows them to adapt to the periodic variations of their environment, and in particular alternating between night and day. In human beings, as for mammals, it is present in the form of a cluster of neuronal cells located in the anteromedian section of the hypothalamus: the suprachiasmatic nucleus. The neurons which make up the latter express ‘clock genes,’ of which a certain number have been identified: PERIOD1 (PER1), PERIOD2 (PER2), PERIOD3 (PER3), cryptocrome1, cryptocrome2, etc. ‘These genes, whose continuous activity has been given a rhythm by a period of around 24 hours, even in a test tube culture, bring about different physiological variations (body temperature, hormone production, heart rate, etc.) in their circadian rhythmicity,’ explains Pierre Maquet.