Le bâillement vu par les peintres
The neuropharmacology of yawning Argiolas A, Melis MR
EEG correlats of yawning during sleep onset JV Laing & RD Ogilvie
 Legendre, R. et Pieron, H.
De la propriété hypnotoxique des humeurs développées au cours d'une veille prolongée
C.R. Soc. Biol. (Paris)
1912, 72: 210-212
Sleep-wakefulness, EEG and behavioral studies of chronic cats without neocortex and striatum: the "diencephalic" cat Villablanca J, Marcus R
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 mise à jour du 31 juillet 2003
Progress in Neurobiology
2000; 62; 379-406
Why we sleep : the evolutionary pathway to the mammalian sleep
MC. Nicolau, M. Akaarir, A. Gamundi, J. Gonzalez, RV. Rial
Laboratori de fisiologia, Universitat des illes Balears, Palma de Mallorca
[...] The phylogenetic appearance of sleep can be approached through a study of the evolution of the vertebrate brain. This began as an undifferentiated dorsal nerve, which was followed by the development of an anterior simplified brain and ended with the formation of the multilayered mammalian neocortex or the avian neostriate.
The successive stages in the differentiation of the vertebrate brain produced, at least, two different waking types. The oldest one is the diurnal activity, bound to the light phase of the circadian cycle. Poikilotherms control the waking from the whole brainstem, where their main sensorymotor areas lie.
Mammals developed the thalamocortical lines, which displaced the waking up to the cortex after acquiring homeothermy and nocturnal lifestyle. In order to avoid competence between duplicate systems, the early waking type, controlled from the brainstem, was suppressed, and by necessity was turned into inactivity, probably slow wave sleep. On the other hand, the nocturnal rest of poikilotherms most probably resulted in rapid eye movement sleep. The complex structure of the mammalian sleep should thus be considered an evolutionary remnant; the true acquisition of mammals is the cortical waking and not the sleep.  
The relationships between REM, non-Rem (WSW) and waking changes with age. Yawn's frequencies and REM sleep evolve in parallel.
The reptiles seern to be not too different from the rest of poikilotherms, thus making acceptable the search of traits of primitive sleep in these animals. They have two types of behavioral sleep: one nocturnal and associated to the circadian cycles of external light and heat, while the other is diurnal and independent, at a first sight, of external factors, although this really means that eventual factors deterrnining it are not easily observable. Surprisingly, these features show their maximal expression in active animals and not in behaviorally sleeping ones.

In relation to REM, the classical mammalian indicators have not been clearly found in reptiles, but the evidence reported in monotremes seems to be definitive in suggesting that REM is, at least, as old as SWS. Finally, some evidence points to a homology between the whole nocturnal sleep of reptiles and the mammalian REM. [...]

The similarity between manimalian SWS and reptilian waking is much clearer than the supposedly obvious between the two waking states. On the contrary, while achieving the same purpose, i.e. maintaining behavioral activity, the waking states are dissimilar in both anatomic and neurological terms. This led to assert the homology (common phylogenetic origin with independence of function) between mammalian SWS and reptilian waking.

One could arrive to the same conclusions in which the main parts of the mammalian brain have been displayed according to their hierarchical relationship to the activity states. The cortex is in the hierarchical top during waking. It produces the most important part of the sensory processing and the behavioral output, while the remaining lower centers only provide support for the necessary accompanying furictions, such as auxiliary movements, homeostatic adjustments and so on. The situation changes during SWS, because the cortex remains inactivated during this state due to diencephalic ascendant inhibition. Therefore, the brainstem (we include the diencephalon, mesencephalon and rombencephalon under this name) lies in the top hierarchical level during this state while the cortex is kept at the bottom. Finally, there is an additional hierarchical change during REM: now the rombencephalon is on top. The cortex is in full activity during REM but it remains disconnected in sensory and motor terms due to rombencephalic inhibitory influence. In consequence, it has been placed second in the hierarchical control during this state. Finally, the brainstern lies in the lower hierarchical level during this state, in correspondence with the low homeostatic capacity of REM. The arrows represent the directions of level change corresponding to the transitions between waking, SWS and REM. In addition to the transitions represented, there is also the one from REM to SWS or from SWS to waking, but not a direct transition from waking to REM.

In consequence, reptiles cannot have a waking of the same type than that of mammals (cortical waking). This does not means that they cannot be vigilant, but that their vigilance is achieved using the same anatomical regions used by mammals to be asleep in SWS. Deleting the cortex causes no functional change; the rombencephalic inhibition travelling downwards would cause inactivity, both in mammals and in submammals. In conclusion, the REM and the inactivity of poikilotherms are both homologous and analogous.
"Embryogenesis wifi repeai phylogenesis in situations kvere embryologicalprocesses occur in a causal-continuity because the evolutive conditions for this sequence to be built must have occurred through sequential continuous modifications- (Horder, 1989). The main lines in the embryogencsis of the nervous system clearly repeats the phylogeny. In consequence, the activity states should repeat the phylogeny, as they maintain a causal continuity with the anatomic structure of the brain. The sleep could be thus a paradigmatic example in the application of the embryogenetic law and the particular features of the rnammalian polygraphic sleep could be unavoidable results of the evolutionary development of the brain. A mammalian waking is only possible, when the neocortex reaches maturity, and this occurs in the last stages of the ontogeny, what was called "the advanced wakefulness".

When the correlation between sleeping time and brain development was recognized, the sleep seemed needed for brain maturation. Many experimental studies have found severe impairments in brain structure and function consequent to deprivation or reduction in the normal amount of REM. But the relation could be inverse: sleep might be the only possible state during stages of low brain development. A controversy between these two points of view is asymmetrical in more than one aspect: first, from a plain philosophical view point, the simplest hypothesis should always be preferred, and is simpler to suppose the unavoidability of REM in a under developed brain in front of a complex (and unknown) REM dependent mechanism of brain arowth and differentiation.

Why an unavoidable bond between REM and brain maturation should have developed? Second, the need of REM for a correct development of the brain has not been discussed in front of the option of REM unavoidability due to low brain capacity. Experimentation should be performed to test the former option in a similar amount as the one performed to demonstrate the latter. Last, but not least, the experiments showing the need of REM for brain development provide with arguments of necessity but not of sufficiency. On the contrary, an undeveloped brain is a sufficient condition to produce a state of reduced behavioral output, as one would qualify the REM. It is a priori clear that a much smaller amount of brain is needed to produce rest, than needed to produce vigilance. However, as total rest is practically impossible in live beings, some kind of output should exist even in an undeveloped brain and this necessary output is what we call the REM signs.

According to the foregoing arguments, the question of why we sleep has lost sense. Most important than their sleep, mammals gained a thick neocortex and a waking with unparalleled features in the animal kingdom. We just sleep while we are not cortically vigilant. The complexity of the mammalian sleep is only a side effect, which probably does not add too much to the environmentally driven rest- activity cycles of simpler animals. According to Williams rules, the sleep does not need to be adaptive by itself as it perfectly fits in the group of byproducts of other truly adaptive changes. [...]

"SWW turned SWS" hypothesis: first, two changes are needed to explain the production of waking, one for the cortical mammalian waking and another one for the striatal waking of birds. No change is necessary to explain the production of SWS as it passed relatively unchanged from being SWW to be transformed in truc SWS (it is the result of the same change which produced waking, it should not be taken into account more than once). Finally, one mutation is needed to explain the possible disappearance of REM in the echidna. In summary, only two, or three at most, changes are needed to explain the evolution of sleep. However, all are known for sure: the first two ones, needed to explain the changes in the telencephalic structure are undeniable. As it has been already explained, the changes in waking and sleep would only be their side results. The third, already explained, is a side result of the speciation process between platypus and echidnas. In summary, the hypothesis defended in this review needs two mutations, the same found for the hypothesis "REM first" but it is fact more parsimonious, because it explains, not only the sleep, but also the waking, an important missing aspect in the other two alternatives.

In addition to showing higher parsimony, the "SWW turned SWS" hypothesis has other additional virtues: we saw how it is well congruent with the phylogenetic law, with the altricial-precocial dimension and also explains the thermogenic correlations of the waking-sleep states. The alternative hypotheses however, provide no reasons to explain why the supposed phylogenetic changes took place. Those theories meet perhaps the requisite of necessariness, but provide no hint at all neither on sufficiency nor on historical unavoidability. Unlike them, for the "SWW turned SWS "theory, it was necessary to produce deep changes in the structure of the nervous system for a reptile to get adapted to live in the dark and these changes were reasonably sufficient to produce the two types of mammalian sleep.
They were unavoidable to gain the nocturnal niche, constrained by the earlier structure of the brain; the only solution was to suppress duplicated sensory and motor systems, but these systems were intermingled with essential homeostatie functions. Therefore, it was impossible to produce their complete annihilation. Achieving a selective block was thus, once again, unavoidable. Some functions of the brainstem had to be suppressed while others should bc maintained. The result was the SWS, a state of high brainstem activity and low behavioral output. The organism ended with two states of rest: the old one, which resulted in the modern REM and the new one, the SWS. This chain of events was necessary to cope with the new demands of a dramatic change in the lifestyle, but it was also, with high verisimilitude, a sufficient cause. The development of a new waking type is a sufficient reason to always block any other less efficient vigilance, i.e., the creation of every new waking type should unavoidably and immediately cause the apparition of a new type of sleep. This reasoning could scem to be gratuitous and unsupported, but it has probably worked more than once. The telencephaIon has changed two times in the history of the vertebrate brain, not only in mammals, but also in birds. We do not know enough about these animals, however, for whatever unknown reasons, they followed an evolutionary path independent from, but parallel to the mammalian one. After developing a new telencephalic striate complex of capacity analog to the mammalian neocortex, they should also have transformed their old reptilian waking in SWS, ending with the same two types of sleep: the appearance of a new type of waking forced, only by itself, the appearance of again a new type of sleep.
Considering the final result of why we sleep, the paradigm defended in this paper affirms that the particular features of the mammalian sleep confer (in principle) no particular advantages over those found in animals just having activity and rest periods. The success of new paradignis depends perhaps only in part on the empirical facts supporting them and mainly on being able in enduring the resistance opposed by the defenders of différent ideas. One should perhaps consider the possibility of unjustified bias and preconceptions to maintain time and resource consuming efforts with extremely low results. This is undoubtedly, the actual outcome of more than 50 research years pursuing the answer to the question on why we sleep. Lets us admit that science knows for sure that many traits of live beings are neutral and without adaptive value, and that the mammalian sleep could just bc another such example. The most parsimonious explanation is that the universality of the activity and rest cycles must have a cause of higher importance than the existence of a complex mixture of electrophysiological features in a particular branch of the animal kingdom. In any case, the burden of the proof should not be demanded to those assuming a continuity in the causes of the sleep, but tothose searching for additional ones.
yawn-rem sleep