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When the first "Jurassic Park" film premiered in 1993, many paleontologists challenged the portrayal of velociraptors hunting after dark, arguing that dinosaurs were strictly diurnal. Decades later, a landmark study from the University of California, Davis overturned that assumption, showing that different dinosaur species were active at varying times of the day and night.

Researchers focused on a unique eye structure—the scleral ring—found in the eye socket of dinosaurs and the modern avian lineage. While mammals lack this bone, birds retain it, and it offers clues about visual capacity. By comparing scleral rings of 164 living birds to those of 33 fossilized dinosaurs and pterosaurs, the team discovered that nocturnal birds possess larger rings relative to their eye size. The same pattern in the fossils suggests that many dinosaur ancestors were active during low light conditions.

Using this comparative approach, the study paints a richer portrait of the Mesozoic era, revealing a spectrum of activity patterns across the dinosaur clade.

Which dinosaurs were diurnal, and which were nocturnal?

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Analysis of the fossil specimens shows that dinosaurs did not adhere to a single temporal niche. Some species, such as pterosaurs and early avian dinosaurs, had relatively small scleral rings, indicating predominantly daytime activity. Massive long‑necked sauropods like Diplodocus displayed medium‑sized rings, suggesting a cathemeral lifestyle—splitting their active hours between day and night to meet high metabolic demands.

In contrast, the iconic Velociraptor exhibited one of the largest scleral rings among the sample, pointing to a nocturnal hunting strategy. In fact, all carnivorous dinosaurs examined in the study appeared to favor nighttime activity, a pattern that challenges traditional depictions. The exception is Tyrannosaurus rex, whose preserved fossils lack intact scleral rings, leaving its activity pattern still unknown.