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.

Primum Sapienti wrote:

> https://www.mdpi.com/2079-7737/11/5/734
>
> Simple Summary
> Mammals evolved from reptiles as a consequence of an evolutionary bottleneck.

Traditional thought is that mammals arose from the same ancestor
that gave rise to reptiles.

https://en.wikipedia.org/wiki/Synapsid

I always thought hibernation must've played a massive role.

Things that are so rare we might as well never think about them, like
supervolcanic eruptions and large meteorite strikes, happen at
pretty regular intervals when viewed on a geologic time scale. In
fact, I've read it claimed that they were more common in the distant
past, happened more often.

BOOM!

In a moment, something can happen which ends summer for a
period of years. Years. The entire planet can be draped in darkness,
the skies choked in clouds of dust/smoke. Being able to sleep
through it would be quite the advantage. Quite.

> Abstract
> Mammals evolved from small-sized reptiles that developed endothermic
> metabolism. This allowed filling the nocturnal niche.

Of course, there's absolutely no reason why it couldn't have worked the
other way around. That, by becoming warm blooded they were able to
exploit periods of the day when cold blooded animals could not be
active.

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https://jtem.tumblr.com/post/690230483640074240