Here's another research article saying the same thing that I mentioned above:
Our data show that (1) juvenile saltwater crocodiles engage in UEC, (2) we can elicit the behaviour in response to visual stimuli and (3) the animals preferentially orient their open eye towards the salient stimulus. These results are broadly consistent with those of
Mathews et al. (2006) on UEC in fence lizards, and with observations from unihemispherically sleeping birds and aquatic mammals (
Rattenborg et al., 2000;
Lyamin et al., 2008). Consequently, these data might suggest that crocodilians engage in unihemispheric sleep. If true, then unihemispheric sleep is likely to have been present in the most recent common ancestor to archosaurs (crocodilians and dinosaurs, including birds), and given the phylogenetic breadth of UEC across reptiles (
Rattenborg et al., 2000), may have evolved even earlier, perhaps in the ancestor to Sauropsids (avian and non-avian reptiles) or in the stem amniote with a subsequent loss of the state in the ancestor to mammals. Alternatively, and equally interesting, is the possibility that UEC first evolved in early Sauropsids for a purpose beyond that of sleeping unihemispherically, perhaps related to focusing visual acuity and attention in a lateralized brain (
Rogers, 2008) and/or eliminating superfluous parts of a visual field in animals with laterally placed eyes (
Mathews et al., 2006). Birds then evolved the ability to enter sleep with the hemisphere associated with the closed eye as a derived feature of sleep. If true, then unihemispheric sleep may have first evolved in the archosaur lineage with the appearance of birds by elaborating upon a pre-existing behaviour inherited from a common ancestor with non-avian reptiles. Distinguishing between these two ideas will require electrophysiological recordings from both brain hemispheres of crocodiles engaged in UEC, and would do much to enlighten our understanding of the evolutionary history of unihemispheric sleep.
Unihemispheric sleep in crocodilians?
Basically just noting a feasibility that unihemispheric sleep actually evolved and was present in animals long before whales even existed.
Which means that such a dilemma in which whales were drowning wouldn't have logically needed to exist, because whales (terrestrial cetaceans) could sleep with half the brain prior to even going into the water.