Much of human health revolves around sleeping well through the night. We know about circadian and body temperature rhythms which make us sleepy at night, peak melatonin production in the middle of the night, deep sleep phases, sleep disruption from light at night, and high cancer rates among people who work the night shift.

Clearly, we are built to sleep at night. You would suppose, then, that we must have evolved to be diurnal long ago and descend from a long line of diurnal animals.

Yet, the evolutionary legacy of mammals is predominantly nocturnal. Mammals started out as nocturnal (to avoid the reptiles) and most mammals today are still nocturnal.

How did we get so staunchly diurnal so quickly?

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    $\begingroup$ You think 65 million years is a short period of time? Part of the answer is that the extinction of the dinosaurs left a lot of daytime ecological niches for previously-nocturnal mammals to fill. $\endgroup$ – jamesqf Jan 20 at 18:38
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    $\begingroup$ jindeed, a short enough time to turn an pig into a blue whale isn't a short time. $\endgroup$ – com.prehensible Jan 20 at 22:53

The Nocturnal bottleneck for mammals as it's called was assumed 'til recently to have only ended at the Cretaceous–Paleogene extinction event 66 million years ago.

At the basis of the nocturnal bottleneck hypothesis lies the idea that Mesozoic ectothermic reptiles (including dinosaurs) would be restricted to daytime activity at times when the sun can help to heat up their body to operating temperatures.......

The arguments against:


  • (i) presence of insulating proto-feathers,

  • (ii) fossils of breeding dinosaurs in cold regions

  • (iii) raised body posture

  • (iv) fibrolamellar bone structures

  • (v) presence of nasal turbulate bone structures

  • (vi) stable growth rate (isotope deposits in bones and teeth)

  • (vii) possible ‘mass homeothermia’ or ‘inert homeothermia’ in large dinosaurs

  • (viii) the occurrence of small dinosaurs in cold regions

  • (ix) the existence of potential external thermoregulatory structures in some dinosaur species (e.g. dorsal boney plates in Stegosauria).

Recent molecular analyses have added to the doubt of the original theory:

saurischians are now generally believed to have had partial endothermia, allowing them to incubate their eggs and inhabit colder regions......

Independently, parallel evolution in Synapsida led to partial endothermia in therapsid reptiles (250 Ma, Late Permian) and full endothermia in mammals

enter image description here

Nespolo RF, Bacigalupe LD, Figueroa CC, Koteja P, Opazo JC. 2011. Using new tools to solve an old problem: the evolution of endothermy in vertebrates. Trends Ecol. Evol. 26, 414–423

The implication being that the pure nocturnal adaptation in mammals would not be a completley effective strategy for predator evasion.

In a large comparative study [Roll], divided 700 species into classes of dominant temporal activity niche: nocturnal, diurnal or both (mostly crepuscular). They used an existing phylogenetic construction to reconstruct the evolution of diurnality in rodents, which are thought to have diversified after the K/T boundary. The reconstruction of temporal niche usage within rodents led to the conclusion that rodents shared a nocturnal ancestor that existed before the K/T boundary. Subsequently, diurnality evolved through secondary evolution at least seven times independently in Rodentia.

Using the molecular phylogenetic construction of [Huchon et al], we constructed the temporal niche phylogeny within the full therian subclass using the nocturnal, diurnal and crepuscular/arrhythmic classification of [Roll et al.]. The most parsimonious interpretation leads to a minimum of 16 changes of dominant temporal niche.

Bolding mine.

Most subdivisions of Theria are indeed nocturnal, but many changes and reversions occurred, and different interpretations may be possible, especially when higher-quality data on activity patterns in more species become available.


It apears that mamals may have been chased by the saurischians throughout the day, night and twilight hours and that many adaptations and reversions may have occured over a period (possibly) much longer than the 66 million years previously thought. Research is ongoing.

Bolded references:

Roll U, Dayan T, Kronfeld-Schor N. 2006. On the role of phylogeny in determining activity patterns of rodents. Evol. Ecol. 20, 479–490

Huchon D, Madsen O, Sibbald MJ, Ament K, Stanhope MJ, Catzeflis F, de Jong WW, Douzery EJ. 2002. Rodent phylogeny and a timescale for the evolution of Glires: evidence from an extensive taxon sampling using three nuclear genes. Mol. Biol. Evol. 19, 1053–1065

  • $\begingroup$ Another point against strictly diurnal dinosaurs is that birds evolved from their dinosaur ancestors long before the K-T extinction event. While most modern birds are diurnal, there are exceptions, notably the owls. Although owl fossils only date back 30-40 million years, there doesn't seem to be any obvious reason why earlier birds, or their dinosaur ancestors, couldn't have evolved a similar lifestyle. $\endgroup$ – jamesqf Jan 21 at 18:02
  • $\begingroup$ @jamesqf 250Ma ago, in fact.There was a bit about sexual selection and owls in the study (I linked to under the word "argument") that you might find interesting: "highly enhanced scotopic vision (e.g. owls), or other senses (e.g. echolocation in bats), may negate such disadvantages. In this respect, it may be hypothesized that the complex sexual selection mechanisms that have arisen through diverse and vivid feather coloration may be the main drive for a diurnal lifestyle where an acute colour visual sensory system would be an advantage." $\endgroup$ – Duckisaduckisaduck Jan 21 at 18:26
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    $\begingroup$ Interesting, yet I wonder if predator evasion is a really a satisfying explanation. After all, many small mammals today are preyed on by birds (among other creatures): hawks in the daytime, owls at night, yet don't seem to be in danger of extinction. (At least from non-human causes.) And many small mammals are burrowers, which means they'd encounter low/no light conditions in daytime. $\endgroup$ – jamesqf Jan 22 at 6:02

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