https://www.mdpi.com/2079-7737/11/5/734
Simple Summary
Mammals evolved from reptiles as a consequence of an evolutionary bottleneck. Some diurnal reptiles extended their activity, first to twilight and then
to the
entire dark time. This forced the change of the visual system. Pursuing
maximal
sensitivity, they abandoned the filters protecting the eyes against the dangerous
diurnal light, which, in turn, forced immobility in lightproof burrows
during light
time. This was the birth of the mammalian sleep. Then, the Cretacic-Paleogene extinction of dinosaurs leaved free the diurnal niche and allowed the
expansion
of a few early mammals to diurnal life and the high variability of sleep traits. On
the other hand, we propose that the idling rest is a state showing
homeostatic
regulation. Therefore, the difference between behavioral rest and wakeful idling
is rather low: both show quiescence, raised sensory thresholds,
reversibility,
specific sleeping-resting sites and body positions, it is a pleasing
state, and both
are dependent of circadian and homeostatic regulation. Indeed, the most important difference is the unconsciousness of sleep and the consciousness of wakeful idling. Thus, we propose that sleep is a mere upgrade of the
wakeful rest,
and both may have the same function: guaranteeing rest during a part of the daily cycle.
Abstract
Mammals evolved from small-sized reptiles that developed endothermic metabolism. This allowed filling the nocturnal niche. They traded-off
visual acuity
for sensitivity but became defenseless against the dangerous daylight. To
avoid
such danger, they rested with closed eyes in lightproof burrows during light-time.
This was the birth of the mammalian sleep, the main finding of this report. Improved audition and olfaction counterweighed the visual impairments and facilitated the cortical development. This process is called “The Nocturnal Evolutionary Bottleneck”. Pre-mammals were nocturnal until the Cretacic-Paleogene extinction of dinosaurs. Some early mammals returned to diurnal activity, and this allowed the high variability in sleeping
patterns observed
today. The traits of Waking Idleness are almost identical to those of behavioral
sleep, including homeostatic regulation. This is another important finding
of this
report. In summary, behavioral sleep seems to be an upgrade of Waking
Idleness.
Indeed, the trait that never fails to show is quiescence. We conclude that
the main
function of sleep consists in guaranteeing it during a part of the daily
cycle.
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