As I lazily looked through my Sunday newspaper (complete with Parade insert), I took the liberty of completing the Numbrix puzzle. In the less than five minutes it took to finish the puzzle, I look just above it to read the Ask Marilyn column. I generally disregard advice and question columns, but I'm aware of Marilyn vos Savant's reputation as a "smart person"(tm) and see the question "Do all animals sleep?".
The reply? "No..."
"Okay," I think, "perhaps she's going to qualify this statement with a rather detailed behavioral definition." (Siegel published a review in Trends in Neuroscience that would agree with the general view, although he's much more careful than vos Savant.)
Well, not exactly. She admits to a behavioral definition, but not a detailed one. And her reply irks me ever so slightly. I'll admit, I've had little respect for her since she decried Andrew Wiles' proof of Fermat's Last Theorem invalid on the basis that it was non-Euclidean. So I suspect my irritation is partly a result of bias, but that doesn't make her reply any more true or correct.
Let's break down her reply:
She begins by noting that she's not including rest or reduced activity - a fair caveat, and a better statement than what follows, although a bit off. I'll let that much slide. But by most accounts, normal rest still is marked by a rather low arousal threshold (similar to the arousal threshold for an obviously vigilant animal). Sleep, not so much. One of the more common operational definitions of sleep in non-mammals is this reduced arousal threshold. This was the point of a recent study in Nature (2008) by Raizen et al., determining that lethargus in C. elegans (the flatworm) was a sleep-like state. For those familiar with the nervous structural changes associated with sleep in animals, it's worth noting that this sleep-like state preceded each new stage of development.
She then cites tuna, and seems to be correct in oxygen demands and water movement over gills (as far as I can tell, since it's not the point of calling her out on this), using the fact of required motion to refute that tuna enter a sleep-like state. The first problem to note here is that sleep is generally not defined by motion or lack thereof in all but the most loose definitions involving mammals. Evidence continues to suggest that sleep is primarily a nervous phenomenon, and the prior mentioned arousal thresholds are sometimes more promising measures. This has been observed in zebrafish (Yokogawa et al., 2007) as well as a rebound of this increased arousal threshold period when deprived it by electrical stimulation.
This same pattern (increased arousal threshold with rebound) is even seen in insects, with the best studied example being Drosophila melanogaster (the fruit fly, a favorite of geneticists since Thomas Hunt Morgan).
Lastly, vos Savant cites what is one of the most unusual mammals in terms of sleep patterns - dolphins. The claim is that dolphins rest half their brain at a time, and are otherwise vigilant. The claim is, at its base, true: dolphin sleep is often unihemispheric. However, I don't think vos Savant can use a mammal, with robust EEG characteristics of sleep, as an example of not-sleep at all. Dolphins are a favorite of sleep researchers actually because of their unihemispheric deprivation potential. I don't think I've ever seen a researcher ever call what dolphins do "not-sleep". As noted earlier, total immobility is not the case for cetaceans... but it's not the case for almost anything aquatic. However, even in cetaceans, reduced activity is associated with sleep, and the extent of activity in captivity is circling the perimeter of the enclosure with minimal echolocation activity (Lyamin et al., 2001). (Note on the paper: the novel research is concerned mostly with gray whales, but the paper is openly accessible and covers dolphins in the introduction.)
So Marilyn is right in a narrow sense - animal sleep is not totally equivalent to human sleep. But of course not! And yet, there is still enough uniformity for researchers to at least call states in animals from dolphins to flies to the simple flatworm as at least "sleep-like". Considering sleep as a nervous phenomenon is likely the most useful characterization of sleep, and considering it as such sweeps away Marilyn's critiques. So it's strictly, yet vacuously true that animals don't undergo human-like sleep. At the same time, it's missing a lot of truth. She constrains herself by her definition from the beginning (not a very good definition, either), and in doing so does a disservice to the concept of sleep in general.
Hendricks JC, Finn SM, Panckeri KA, Chavkin J, Williams JA, Sehgal A, & Pack AI. (2000) Rest in Drosophila Is a Sleep-like State. Neuron 25(1):129-138 doi:10.1016/S0896-6273(00)80877-6
Lyamin, Manger, Mukhametov, Siegel & Shpak. (2001) Rest and activity states in a gray whale. Journal of Sleep Research 9(3):261-267. doi:10.1046/j.1365-2869.2000.00212.x
Raizen DM, Zimmerman JE, Maycock MH, Ta UD, You Y-j, Sundaram MV & Pack AI. (2008) Lethargus is a Caenorhabditis elegans sleep-like state. Nature 451:569-572 doi:10.1038/nature06535
Yokogawa T, Marin W, Faraco J, Pézeron G, Appelbaum L, et al. (2007) Characterization of Sleep in Zebrafish and Insomnia in Hypocretin Receptor Mutants. PLoS Biol 5(10): e277. doi:10.1371/journal.pbio.0050277