Are any animals nocturnal?

Question

Several animals are commonly considered to be 'nocturnal' by nature.

Nocturnality is an animal behavior characterized by activity during the night and sleeping during the day. The common adjective is "nocturnal".1

Some examples include owls and hedgehog. Indeed, the very existence of such a word implies it is used to describe some observed behavior.

Some quotes from wikipedia on owls:

Most owls are nocturnal, actively hunting their prey only in darkness.2

And on Hedgehog

Hedgehogs are primarily nocturnal, although, depending on the species, they may be more or less active during the day.3

However, isn't it more likely that they simply follow a polyphasic sleep schedule?

Polyphasic sleep, a term coined by early 20th-century psychologist J.S. Szymanski, refers to the practice of sleeping multiple times in a 24-hour period—usually more than two, in contrast to biphasic sleep (twice per day) or monophasic sleep (once per day).4

It seems rare for any animals to sleep for particularly long periods of time, such as a full night. Even some house cats in a safe controlled environment will usually not sleep for more than a few hours at a time.

I find it unlikely that most animals could survive in the wild if they were truly "nocturnal". Is it possible that their classification as such is simply a result of seeing them active at night, when in fact they may sleep equally during the day?

Bats? Don't they forage at night and only roost during the day? — Larry OBrien, Jun 26 '13 at 20:05
Mice and rats are another example of (very well studied) nocturnal animals. This however, does not necessarily imply they sleep all day. For instance see: http://www.sciencedirect.com/science/article/pii/0006899377900336 — nico, Jun 26 '13 at 20:53
Why on earth do you find it unlikely that nocturnal animals can survive in the wild? — Rory Alsop, Jun 26 '13 at 21:14
This question is about the definition of "nocturnal," and thus better answered by a dictionary or biology text book. — Flimzy, Jun 26 '13 at 23:50
@Larry: This expert claims that [bat's don't sleep continuously](http://www.youtube.com/watch?v=_gLVh3ihRZw) throughout the day. — Oddthinking, Jun 27 '13 at 01:30
@Oddthinking I was thinking that if bats didn't sleep while away from their roosts, and they (typically) stay away from their roosts through the night, that would qualify them as "nocturnal." But as Flimzy says, it all seems to turn on how one defines "nocturnal." — Larry OBrien, Jun 27 '13 at 01:39
@Flimzy I thought the question assumed the definition of nocturnal as "asleep during the day, awake at night". — , Jun 27 '13 at 04:15
@Sancho: The question appears to assume a definition of "never awake at during the day", which doesn't seem to fit any accepted definition. — Flimzy, Jun 27 '13 at 04:45
@Flimzy But the question explicitly says: "'nocturnal' (asleep during the day, awake at night)" — , Jun 27 '13 at 05:58
@Sancho: Yes, but that definition is an over-simplification of what nocturnal normally means. — Oddthinking, Jun 27 '13 at 06:57
Added links to wikipedia to explain all terminology. Fixed the broken facts. (cats and ferrets are not considered nocturnal), but instead used some other animals that are considered nocturnal. — Wertilq, Jun 27 '13 at 08:57
@Oddthinking Do we not defer to what the asker means by their question? By changing the definition, or using one different from the one the asker uses, are we not changing the meaning of the question? It seems the asker wanted to know if any animals are "asleep during the day, awake at night", and now, we aren't asking that question. — , Jun 27 '13 at 13:27
@Sancho: Where the term was introduced in the claim, we should use the definition provided by the claimant. That's tricky, because we often can't interrogate the claimant about their definitions. However, a key reason for links to claims is to be able to go to the source, read the original statements and clear up misunderstandings. — Oddthinking, Jun 28 '13 at 02:33
@LarryOBrien there are bats that are active in daylight hours. Fruitbats for example (and I've the photographs to prove it). — jwenting, Jun 28 '13 at 05:31
I think @Wertilq's edits improve the question greatly, but the question should still be off-topic for Skeptics, and instead belongs on a Biology Q&A/Forum site. — Flimzy, Jun 30 '13 at 01:59

Oddthinking · Accepted Answer · 2013-06-27T01:50:39.043

This boils down to an argument about definitions. You argue that brief awakenings during the day means an animal is not truly nocturnal. Turning that around, that would mean awakening during the night would mean an animal is not truly diurnal.

(I am going to restrict the discussion to mammals only. I think the point about definitions still applies to other animal classes that sleep.)

In fact, it is typical for mammals to have brief awakenings during the sleep cycle no matter when it is.

Although mammals of different species have different sleep patterns, brief sleep–wake transitions commonly are observed across species and appear to occur randomly throughout the sleeping period. [...] We analyze sleep recordings from mice, rats, cats, and humans, and we compare the distributions of sleep and wake episode durations. For all four species, we find that durations of brief wake episodes during the sleep period exhibit a scale-free power-law behavior with an exponent α that remains the same for all species (α ≈ 2.2).

You could conclude that all mammals are cathemeral, but such a strict definition removes the useful distinctions that nocturnal, diurnal and crepuscular offer. So, it is reasonable to use a looser definition and describe a species of bat as nocturnal, even though they are sometimes awaken during the day.

Are any animals nocturnal?

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