A prior study in the Journal of Morphology by the pair that showed that the size of the "scleral ring" - a ring of bone that covers the iris and pupil in many animals - is a good indicator of the size of the pupil.
"We measured living lizards and birds to test if you can use these bone parts to distinguish the nocturnals from diurnals," Dr Schmitz explained to BBC News.
"We picked species for which we know their activity patterns - we know if they are nocturnal, diurnal or both - and then we found we could tell them apart [on the basis of their ocular bone sizes].
"Once we had that established we applied what we learned from the living species to the dinosaurs."
The pair measured the eye socket size and scleral ring size of 33 dinosaur fossils, finding that dinosaurs seemed to be busy at a variety of times of day and night.
"Contrary to what was believed - that most dinosaurs were diurnal - we saw pretty much everything: we had diurnal, nocturnal and species active both day and night, that was the first surprise," Dr Schmitz said.
"The activity patterns depend essentially on what they do for a living - what the ecology is. Small carnivores like velociraptor tended to be nocturnal or active day and night; we didn't have a diurnal carnivore in our analysis.
"Large herbivores, they were almost exclusively day- and night-active."
Dr Schmitz said this was probably because larger animals needed to forage longer to support their large sizes.
The find also casts doubt on the idea that mammals - who in modern times are predominantly nocturnal - evolved their activity patterns to avoid being targeted by day-active dinosaurs.
"We now know that dinosaurs were active at night as well, so the whole story is much more complicated," Dr Schmitz said.
"We don't even know if early mammals were truly nocturnal; we have do a lot more research to understand how nocturnality in mammals evolved."
For more information related to dinosaurs, visit rareresource.com.
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