Why we sleep: the evolutionary pathway to the mammalian sleep
Section snippets
Introduction: the mystery of the sleep
For lay people, sleep provides with simple rest and recovery, but the magical world of dreams maintains, in the break point of a new century, a halo of mystery. For sleep researchers, and even for general scientists, the mystery increases: the brain does not rest, even in the deepest sleep. Two types of sleep have been recognized in mammals (Fig. 1), each one with many improbable features: the slow wave sleep (SWS), consisting in a general slowing of most bodily functions, and in a diminished
The causes of the sleep
The bizarre characteristics of the mammalian sleep, the discovery of two extremely different phases, the imperative need of sleep overwhelming the mere need of rest, raise the question on the causes of the sleep, a central issue in sleep research (Horne, 1988). Causal questions in behavior can be interpreted in several ways (Amlaner and Ball, 1994, Timbergen, 1963). A given behavior can appear as the result of some stimulus appearing either in the environment or in the internal medium, and in
The sleep as adaptation
A fundamental idea pervades every report proposing a physiological function for the sleep: its fundamental aim is a quest for adaptiveness. Every researcher interested in recognizing the function of the sleep should thus be an adaptationist at heart. If the sleep has any purpose at all, sleeping organisms should have an advantage in being sleepers in front of other hypothetical beings devoid of sleep. As animals do sleep, the trait should have been selected as better adapted than other eventual
Behavioral and polygraphic sleep
The search of the sleep causes presents additional problems due to the difficulty in finding a general definition of sleep. Many authors have contributed to develop a definition of sleep as a behavior. Almost 100 years ago, Pieron (1913) proposed three characteristics: (1) motor rest, (2) increased sensory thresholds and (3) easy reversibility. Flanigan (1973) added (4) stereotyped posture. Bruce Durie (1981) also included (5) specific rest sites and (6) circadian organization. This author also
Searching the evolutionary origin of sleep and waking
Sadly, sleep leaves no fossil record and the only way to understand its evolution will be of comprehensive nature, analyzing its manifestations in surviving forms. This has been attempted several times (Allyson and Van Twiver, 1970, Tauber, 1974, Monnier, 1980, Karmanova, 1982, Meddis, 1983, Hartse, 1994), but curiously, the evolution of waking has only been recently considered (Rial et al., 1993, Rial et al., 1997); the unity of the waking state in all vertebrates has been taken for granted.
Does the behavioral sleep in reptiles exist?
A number of reports have addressed the question of the activity states in reptiles. Most of these studies were made around the 70’s and almost all described the existence of a well organized behavioral sleep (Hunsaker and Lansing, 1962, Hermann et al., 1964, Tauber et al., 1966, Tauber et al., 1968, Vasilescu, 1970, Peyreton and Dusan-Peyreton, 1969, Karmanova and Churnosov, 1972, Flanigan, 1973, Flanigan, 1974, Flanigan et al., 1974a, Flanigan et al., 1974b, Hartse and Rechtschaffen, 1974,
The reptilian neurophysiology
If reptiles have a behavioral sleep more or less similar to the mammalian one, what can be said about its neurophysiological correlates? Trying to solve this question several studies have attempted to find slow EEG waves (or some alternative), spindles, and K-complexes to define SWS in reptiles. In the same way, the traits of REM in the form of rapid eye movements or low voltage and mixed frequency EEG, were looked for. The following sections summarize the main results.
The meaning of the differences in the mammalian and reptilian state indicators
Having described the most salient facts on the reptilian neurophysiology, one could try to compare their characteristics with those of mammals. This is shown in Table 1, where the neurophysiological characteristics of waking reptiles described up to now are compared with those of waking and SW sleeping mammals. Taking cortical activity, as shown by the EEG power profile, arousal reaction and evoked potentials, waking reptiles are opposite to waking mammals and practically equivalent to SW
How the mammalian waking and sleep appeared
Between late Triassic and the end of Cretaceous, reptiles were the dominant land vertebrates (Crompton et al., 1978). As the extant reptiles, they would only show two different behavioral states: activity during daytime and rest during night, constrained by the need of external heat sources. Only big animals, with high thermal inertia, would be able to maintain a relatively constant body temperature and night activity (Desmond, 1975). These exceptions aside, the night would be devoid of active
Why do we sleep?
The evolutionary path drawn in this way correlates well with the evolution of the vertebrate brain, with its hierarchical structure (Parmeggiani, 1982) (Fig. 9), and with the ontogeny of wakefulness and the two sleep states (Roffwarg et al., 1966). Nobody would consider the recapitulation law of Von Bauer and Haeckel as an absolute fact, because there is the possibility of a brand new feature, unrelated to phylogeny, appearing in the embryo of any species. However, it is striking how the law
The mammalian sleep as secondary adaptation
Indeed, additional environmental pressures may have subsequently modified the sleep in many ways. Evolutionary remnants can be reused for new purposes and several utilities might exist for the sleep in different animal species (Horne, 1988). Sleep may even turn to extremely reduced amounts when the predation or other environmental pressure turns the sleeping behavior lethal or semi lethal (Allison and Chichetti, 1976) or it can turn asymmetric in bizarre mammals having returned to life in the
Discussion
Three theories have been developed to explain the evolution of the sleep. First, Tauber et al., 1966, Tauber et al., 1968, Tauber et al., 1969, proposed that REM should be considered the primitive sleep. Their hypothesis will be called henceforth the “REM first” one. Almost simultaneously, the opposite hypothesis (“SWS first”) was proposed by Allison and Collaborators (Allyson and Goof, 1968, Allyson and Van Twiver, 1970, Allyson et al., 1972). This point of view received strength after the
Acknowledgements
This work has been in part supported by grants of the DGICYT, PS-93-0421 and FIS, 97/1032 from the Spanish Government.
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