Are the slow waves which are characteristic of slow deep sleep simply, as has been thought up to this point, overseeing a period of “rest” for our brain as it “recuperates” after the intense activity of wakefulness? A study undertaken by researchers at the University and the Liège CHU, published in the American journal Proceedings of the National Academy of Sciences (PNAS), show that they orchestrate a ballet of cerebral activity comprised of “ups” and “downs”.
Since the 1950s, it has been known that human sleep consists of two major stages: slow sleep and paradoxical sleep. Paradoxical sleep, which produces dreams, is associated with metabolic energy levels which are as high as during wakefulness, while the former, slow sleep, which takes up the majority of our sleep at the beginning of the night, has been likened to a “little death”, in particular in phases 3 and 4, which has come to be known as slow deep sleep. Glucose consumption in the brain is reduced to 30-40% of its waking levels and cerebral blood flow reduces. According to classical theory, slow deep sleep is a period of quiescence “offered” to the brain by means of compensation for its earlier intense wakefulness; it is a period where cerebral tissue is restored by mechanisms as yet unknown.
On an encephalogram, slow deep sleep is characterised by slow waves with a frequency of less than 4 hertz, which are known as slow oscillations. But are they really there, as it has been believed until now, to oversee a period of cerebral quiescence, devoted to absolute calm? In a study published in the prestigious American journal Proceedings of the National Academy of Sciences (PNAS), a team bringing together researchers from the University of Liège Cyclotron Research Centre (CRC) and the Liège CHU Neurology Department demonstrated that it is nothing of the sort.
Other research groups have previously demonstrated significant neuronal activity in a cat in slow deep sleep state. Indeed, in the brain of an animal in deep sleep, the activity of large populations of neurons is synchronised by slow oscillations which is shown by the alternating periods of phases of discharge and of silence. During wakefulness the majority of neurons are, on the whole, silent and only “fire” to support a physiological function or motor, cognitive or other activity. In contrast, during slow deep sleep, the cat’s neuron populations oscillated over a short period of time between one state characterised by membrane hyperpolarisation – a condition which forces them to be silent – and another, where the said membrane is depolarised – in other words, the neurons discharge. Specialists speak of a succession of “down states” and “up states”. These states do not occur randomly, but are regulated by the slow oscillations characteristic of slow deep sleep: “down” when the wave approaches a trough, “up” when it rises towards its peak (2).
(1)a T.T. Dang-Vu, M. Schabus, M. Desseilles, G. Albouy, M. Boly, A.
Darsaud, S. Gais, G. Rauchs, V. Sterpenich, G. Vandewalle, J. Carrier,
G. Moonen, E. Balteau, C. Degueldre, A. Luxen, C. Phillips and P.
Maquet, Spontaneous Neural Activity during Human Slow Wave Sleep, PNAS,
30 sept. 2008, vol. 105, n° 39, 15160-15165.
(2) This game of “ups” and “downs” is also seen in other circumstances: in patients under anaesthetic, for example.