Sleep Medicine Reviews (2007) 11, 411–417 REPLY TO INVITED COMMENTARY Sleep and wakefulness, trivial and non-trivial: Which is which? Rube´nV.Rial à , Marı ´a C. Nicolau, Antoni Gamundı ´, Mourad Akaa ˆrir, Sara Aparicio, Celia Garau, Silvia Tejada, Catalina Roca, Lluis Gene´, David Moranta, Susana Esteban Laboratori de Fisiologia, Institut Universitari de Cie`ncies de la Salut, Universitat de les Illes Balears, 07122 Palma de Mallorca, Spain Introduction Our main interest in writing a review on the trivial nature of sleep 1 was asserting that the main function of sleep is merely adapting the activity of the organism to the light-dark cyclic changes of the planet but also to confront the triviality of sleep with the non-trivial functions of wakefulness. We are happy to see that our viewpoint has provoked a discussion on many aspects of sleep which in our opinion are taken for granted but actually lack sufficient objective support. We are thus grateful to Rattenborg and co-workers 2 for having accepted the invitation to criticize our review. We examine next these criticisms. The regulation of rest in poikilotherms In our review, we proposed that the evidence for rest regulation in poikilothermic animals is insuffi- cient and far from conclusive. On the contrary, there is an overwhelming amount of data showing that poikilotherms could persist resting immobile during long periods. For instance, every amateur herpetologist knows that frogs, turtles, snakes and lizards can spend weeks and even months in almost total inactivity, without signs of discomfort and without harmful effects. This means that their rest is not regulated and what is really important is their activity time. The discrepancy between these widely known facts and the experimental data obtained in several sleep laboratories should be due to mistakes introduced either in the design or in the interpretation of the results. In our opinion, a part of the problem lies in ignoring the effects not only of fatigue, which at least in humans has been found to be independent of sleepiness, 3 but also of stress. We believe that sleep deprivation without stress contamination has been produced, if any, in a very limited number of cases. However, experi- ments in poikilothermic animals could have been flawed in a number of additional ways. For instance, certain experiments in Drosophila 4 ob- served, that periods of immobility greater than 5 min resulted in decreased responsiveness to mechanical stimulation. This was interpreted as sleep-provoked raised thresholds. However, this result is different from others 5,6 which found that the Drosophila responsiveness to most sensory systems is inversely related to the overall level of arousal. Nevertheless, insects suffer activity-re- lated variations in body temperature 7 even in a constant temperature environment. 8 Thus, regard- ing the counterproofs argued by Rattenborg and ARTICLE IN PRESS www.elsevier.com/locate/smrv 1087-0792/$ - see front matter & 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.smrv.2007.06.001 à Corresponding author. E-mail address: rvrial@uib.es (R.V. Rial).