Author: Ruben Rial
Position: Professor
Institution: Universitat de les Illes Balears
E-mail: rvrial@uib.es
Additional Authors: Maria Nicolau , Antoni Gamundi , Mourad Akaârir , Sara Aparicio , Celia Garau , Silvia Tejada , Lluis Gené , David Moranta , Susana Esteban
Submitted Date: November 14, 2007
Published Date: November 14, 2007
This comment was originally posted as a “Reader Response” on the publication date indicated above. All Reader Responses are now available as comments.

Sleep could be defined in behavioral terms as a reversible state with motor rest, increased sensory thresholds, stereotyped body position, specific sleeping places and circadian organization [1-3]. The report of Yokogawa et al [4] on the zebrafish sleep provides strong evidence of these signs. However, these signs are under the control of circadian and homeostatic mechanisms [5,6,7] during sleep, which should be recognized consequently as essential to define the presence of sleep in a given animal.

The observation of cyclic behavior during extended time periods in constant conditions is essential to ascertain the presence of circadian cycles. This condition has not been fulfilled in the experiments reported, as “the sleep” of the zebrafish was only studied during two days in DD and three days in LL conditions. If we analyse these (too short) observations, the differences corresponding to subjective day and night seem to be rather small and probably lacking statistical significance (Table 1 in [4]). This contrasts with the strong variations between day and night observed in animals submitted to 14:10 LD. However, this only demonstrates the powerful masking effects of the environmental light. Therefore, we see no evidence of circadian control in the report.

The evidence is also insufficient for homeostatic regulation. Indeed, experimental and control groups behaved differently during deprivation and showed different consequences after six hours of extended activity. However, the post deprivation behavior could be a true sleep, but also fatigue after the extended activity or any other use dependent factor. The absence of rest rebound observed in the light period after deprivation recovery greatly increases the doubts on the presence of true sleep.

Considering that the circadian activity-rest cycles are universal in the animal kingdom and that their adaptive value is undisputed [8], we are confident that evidence for the circadian organization of time in zebrafish will be eventually obtained. Thus, we will not dispute the existence of such regulation in the zebrafish in spite of the null evidence shown in [4]. However, granting the circadian regulation alone, would only serve to recognize the existence of rest/activity cycles but not of sleep, which is essentially dependent upon homeostatic regulation [5,6,7]. As such evidence is lacking, we doubt that the state described in [4] could be called sleep. Strictly speaking, and leaving apart the interesting results reported in relation to the activity of hypocretins, the only sleep related consequence of the study consists in the presence of rest and activity periods tightly bound to environmental light and, indeed, to other external factors, such as the sensory stimulation. This conclusion adds evidence to the assertion that true sleep is absent in poikilothermic animals [9].

References
1. Pieron, H. (1913) Le problème physiologique de sommeil. Paris, Masson. 520p.
2. Flanigan, WF Jr. (1973) Sleep and wakefulness in iguanid lizards, Ctenosaura pectinata and Iguana iguana. Brain Behav Evol 8:401-436.
3. Bruce Durie DJ (1981) Sleep in animals. In: Wheatley D. editor. Psychopharmacology of sleep. New York, Raven Press. pp 1-18.
4. 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 Biology 5(10):2379-2397.
5. Borbély AA. (1980) Sleep: circadian rhythm versus recovery process. In: Koukkou M, Lehmann D. Angst J. Editors. Functional states of the brain: their determinants. Amsterdam, Elsevier. pp. 151 161.
6. Borbély AA (1982) A two-process model of sleep regulation. Hum Neurobiol 1:195–204.
7. Tobler I. (2005) Phylogeny of sleep regulation. In: Kryger MH, Roth T, Dement WC, editors. Principles and practice of sleep medicine. Philadelphia, Elsevier/Saunders pp. 77-90.
8. Aschoff J. (1964) Survival value of diurnal rhythms. Symp Zool Soc London 13:79–98.
9. Rial RV, Nicolau MC, Gamundí A, Akaârir M, Aparicio S, et al (2007) The trivial function of sleep. Sleep Medic Rev 11:311–325.

Acknowledgements
This work has been in part supported by grants of the DGICYT, PS-93-0421 and FIS, 97/1032 from the Spanish Government.