The Birth of the Mammalian Sleep

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.

Keywords: evolution of sleep; evolutionary bottleneck; function of sleep; sleep variability; wakeful idling.

Figure 1

Phylogenetic three of amniotes. At present, three main branches exist: modern reptiles, birds-(including crocodiles) and modern mammals, that appeared after a nocturnal bottleneck.

Figure 2

A 30 g alert Gallotia galloti lizard in RAB attitude after heliothermic warming.

Figure 3

Thermographies of the same lizard of Figure 1. (Left): at noon, the heliothermic warming (BT = 30.2 °C, cursor 2) allowed full activity. (Right): thermography of the same lizard taken in total darkness, during midnight. The lizard had to be passively warmed during a few minutes before taking the right side image. Otherwise, its BT would be identical to that of the background and the animal would be completely invisible under the infrared camera. Nevertheless, despite the increase in BT (25.7 °C, cursor 1), the animal remained immobile in a cataplexic state, making no escape attempts. We attribute the immobility to circadian-related dormancy. Lateral color bars: left, from 30.2 to 17.5 °C. Right, from 27.5 to 22.2 °C. Thermographies taken by an Infrared Irisys IRI 4010 Camera.