Dolphins and a few other species have developed unihemispheric (one-sided) sleep, a remarkable specialization strongly suggesting that sleep must have some essential function and cannot be eliminated [123]. Yet, the very presence of sleep in dolphins has recently been questioned based on four lines of evidence. (1) It has been argued that unihemispheric sleep is not really sleep because dolphins move and, more crucially, because there is no evidence that the hemisphere with slow waves is less capable of responding to the environment [7]. In fact, the presence of slow waves in one hemisphere is associated with unilateral slow waves in the thalamus, and unilateral decrease in brain metabolism, including in the ipsilateral locus coeruleus, an arousal-promoting system [124]. In other words, electrophysiological and metabolic processes that always occur bilaterally in the brain of other mammals can be engaged unilaterally in the brain of a dolphin. It therefore seems at least plausible that half of the dolphin brain can be unresponsive while the other half may be awake. Indeed, there are a few brief reports showing that both bottle-nosed and white-sided dolphins show reduced or no response to stimuli when performing stereotypical circular swimming, which is associated with unihemispheric sleep [122,125].
(2) Another argument was raised by a report describing continuous activity in newborn dolphins (and whales) and their mothers for the first postpartum month [126]. However, based on 2 additional recent studies that assessed eye closure under water [127,128], it seems likely that young cetaceans engage in unihemispheric sleep while swimming, and do so from birth for many hours a day. It has been argued [7] that even if present, this unihemispheric sleep could not be restorative, because it is interrupted every 30–60 s by breathing. The basis for the latter assumption remains unclear. Rats sleep 12–14 h per day and their sleep cycle lasts approximately 10–20 min. When forced to a schedule of 30-s stimulation on/90-s stimulation off, rats learn quickly to sleep in the 90-s off period, so that their total daily time in NREM sleep does not change (and the intensity increases [61]). When tested in a spatial learning task, these animals, which are not capable of unihemispheric sleep, still perform at 70%–80% of baseline levels. Until (if ever) combined EEG-performance studies will be performed in young cetaceans, it seems at the very least premature to imply that their sleep must necessarily be poor and not restorative. Even so, the issue is not how well they sleep, but whether they sleep.
(3) Regarding sleep homeostasis, there is only one publication [45] in which EEG recordings were used in dolphins to measure the response to sleep deprivation. Of note, the study used different lengths of sleep deprivation (35–150 h) and of recovery (9–24 h), slow waves could not be totally prevented during the sleep deprivation, recovery ended at different time of day, data were averaged for the entire recovery period, and only sleep duration (not sleep intensity) was measured. Nevertheless, it was found that (i) during sleep deprivation the amount of stimulation required to prevent slow waves increased progressively; (ii) in all cases (n = 6) bilateral sleep deprivation increased sleep time during recovery; (iii) in all cases (n = 9) unihemispheric sleep deprivation increased sleep duration in the affected hemisphere. Though the results of this seminal study have been characterized as “very variable” [7], it is hard to deny the presence of a clear-cut sleep rebound.
(4) A final issue was raised by a recent study in two highly trained dolphins, which showed that they could maintain continuous vigilance for 5 d [125]. In fact, during the stimulation period the two animals displayed resting behavior at night (floating or very slow stereotyped swimming), and response times were slower at night than during the day, suggesting that at least some rest was obtained (most likely unihemispheric sleep, as suggested by the authors). Moreover, one of the two dolphins the last night “ignored all target stimuli for 4 hours and appeared to be asleep” [125].