Unearthing the phylogenetic roots of sleep

Abstract

Why we sleep remains one of the enduring unanswered questions in biology. At its core, sleep can be defined behaviorally as a homeostatically regulated state of reduced movement and sensory responsiveness. The cornerstone of sleep studies in terrestrial mammals, including humans, has been the measurement of coordinated changes in brain activity during sleep measured using the electroencephalogram (EEG). Yet among a diverse set of animals, these EEG sleep traits can vary widely and, in some cases, are absent, raising questions as to whether they define a universal, or even essential, feature of sleep. Over the past decade, behaviorally defined sleep-like states have been identified in a series of genetic model organisms, including fish, flies and worms. Genetic analyses in these systems are revealing a remarkable conservation in the underlying mechanisms controlling sleep behavior. Taken together, these studies suggest an ancient origin for sleep and raise the possibility that model organism genetics may reveal the molecular mechanisms that guide sleep and wake.

Figure 1

Mammalian phylogenetic order is not strongly correlated with sleep parameters. Despite similar genetics and physiology, sleep times within mammalian orders overlap extensively. On the left are three pairs of animals that are in the same order but have very different sleep parameters. On the right are three pairs of animals from different orders with similar sleep amounts. Mammalian sleep times are not strongly correlated with phylogenetic order [1,13,33]. Photo of Eastern American Mole, courtesy Barbara L. Clauson and Robert M. Timm; the ajacent photo of J.M.S., courtesy of N.Y. Times. Photo of R.A., courtesy of Northwestern University. Photos of other animals courtesy of Wikipedia commons site.

Figure 2. Fish, flies and worms

(A) The adult zebrafish, Danio rerio. (B) The adult fruit fly, Drosophila melanogaster. (C) The adult nematode, Caenorhabdities elegans. Photos courtesy of Wikipedia Commons website.

Figure 3. The Drosophila activity monitoring system

The top panel is a schematic of a behavioral assay tube with an agar food plug at one end. The tube is crossed by an infrared beam. The bottom panel is a photograph of an activity monitor. A U.S. penny (19 mm) is indicated for scale. A fruit fly is indicated by an arrow.