Everybody knows that sleep is important, yet the function of sleep seems like the mythological phoenix: “Che vi sia ciascun lo dice, dove sia nessun lo sa” (“that there is one they all say, where it may be no one knows,” Wolfgang Amadeus Mozart and Lorenzo da Ponte [1790], Così fan tutte). But what if the search for an essential function of sleep is misguided? What if sleep is not required but rather a kind of extreme indolence that animals indulge in when they have no more pressing needs, such as eating or reproducing? In many circumstances sleeping may be a less dangerous choice than roaming around, wasting energy and exposing oneself to predators. Also, if sleep is just one out of a repertoire of available behaviors that is useful without being essential, it is easier to explain why sleep duration varies so much across species [ 1–4 ]. This “null hypothesis” [ 5–7 ] would explain why nobody has yet identified a core function of sleep. But how strong is the evidence supporting it? And are there counterexamples?

So far the null hypothesis has survived better than alternatives positing some core function for sleep [ 8–10 ]. In what follows we shall test the null hypothesis by considering three of its key corollaries. If the null hypothesis were right, we would expect to find: (1) animals that do not sleep at all; (2) animals that do not need recovery sleep when they stay awake longer; and, finally, (3) that lack of sleep occurs without serious consequences.

Corollary 1: Are There Animals That Do Not Sleep?

Sleep is a reversible condition of reduced responsiveness usually associated with immobility. The decreased ability to react to stimuli distinguishes sleep from quiet wakefulness, while its reversibility distinguishes sleep from coma. Only a small number of species—mostly mammals and birds—have been evaluated in detail with respect to sleep. Most studies found signs of sleep, both behavioral (quiescence and hyporesponsivity) and electrophysiological (e.g., the slow waves of non-rapid eye movement [NREM] sleep). Scientists have been hesitant to attribute sleep to reptiles, amphibians, fish, and especially invertebrates, preferring the noncommittal term “rest” in the absence of electrophysiological signs resembling those of mammals and birds. Studies with Drosophila melanogaster [11,12], however, demonstrated that flies, also, become less responsive, i.e., sleep, when they remain quiescent for a few minutes. Moreover, sleep pressure increases if flies are kept awake, their sleep patterns change with the life span, and they are sensitive to hypnotics and stimulants [13–15]. Finally, the fly brain undergoes changes in gene expression between sleep and wakefulness similar to those observed in mammals [16,17], and shows changes in brain electrical activity [18]. Similar criteria have now been provided for zebrafish [19–21], and there is evidence that even the worm C. elegans shows a sleep-like state at a certain stage of development [22].

It has been argued that the assumption that sleep is universal is based on poor evidence [7]. Figure 1 summarizes some of the “difficult” cases. The bullfrog is often promoted as an example of an animal that does not sleep. There is, however, only one study on this topic, published in 1967 [23]. This report concluded that bullfrogs do not sleep because even during the resting phase they never failed to show a change in respiratory responses after painful stimuli (cutaneous shock). The same report acknowledged that arousal thresholds could not be measured during the cyclic phases with the lowest respiratory activity, nor could they be tested with other physiological stimuli, such as light or sound. Also, the underlying assumption in that study was that shocks delivered late at night (presumably in the middle of sleep) should elicit less respiratory response than those given early in the night (when sleep had just started); however, the opposite was found [23]. In fact, we now know that in rodents and humans the deepest sleep occurs early after sleep onset. At the very least, it seems that more experiments are needed before concluding that bullfrogs do not sleep.

Coral reef teleosts showing sleep swimming have similarly been used as evidence that not all animals sleep (Figure 1). Two types of reef fish have been studied in terms of sleep; one is immobile at night and less responsive to alerting stimuli (stationary sleep [24]), and another [25] retreats to the coral at night, where it continues to move its fins even when holding a fixed position (called “sleep swimming”; possibly to avoid hypoxia [25]). The researchers who studied these teleosts defined sleep swimming as a state “equivalent to sleep.” They assumed that sensory information must still be processed to a certain extent during sleep swimming, because each individual remains in its swimming zone during the night. Yet, the fish at night loses the ability to respond to predators [25], and mortality due to predators' attacks is much higher at night, when the fish is sheltering in corals, than during the day, when it feeds in open waters [26]. Most losses to predators occur in the first 1–2 h after sunset, i.e., at the beginning of the “rest” period. Although limited, the available evidence seems to suggest that sleep swimming is associated with hyporesponsivity.

In dolphins the very presence of sleep has been called into question because these marine mammals move continuously and their arousal thresholds have not been measured directly (Figure 2). Yet, dolphins are capable of engaging in slow waves with half of the brain at a time, a property called “unihemispheric sleep” [27–31]. Moreover, there is some limited evidence of decreased response to stimuli during stereotypical circular swimming, which is associated with unihemispheric sleep (Figure 2). The very fact that dolphins have developed the remarkable specialization that is unihemispheric sleep, rather than merely getting rid of sleep altogether, should count as evidence that sleep must serve some essential function and cannot be eliminated. Thus, there is no clear evidence of a species that does not sleep.