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?

  • 2
    Bats? Don't they forage at night and only roost during the day? 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: sciencedirect.com/science/article/pii/0006899377900336
    – nico
    Jun 26 '13 at 20:53
  • 4
    Why on earth do you find it unlikely that nocturnal animals can survive in the wild?
    – Rory Alsop
    Jun 26 '13 at 21:14
  • 9
    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
  • 1
    @Sancho: Yes, but that definition is an over-simplification of what nocturnal normally means.
    – Oddthinking
    Jun 27 '13 at 6:57

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.


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