The Phylogeny of Sleep Database: A New Resource for Sleep Scientists

Keywords: Phylogeny, comparative analyses, REM, NREM, evolution

Validity of Sleep Quotas in Comparative Analyses

Inter-specific variation in time devoted to each of the two major mammalian sleep states (REM and NREM) is a physiologically meaningful index of sleep expression, sleep function and sleep dysfunction. When deprived of the sleep amount they normally require, most of the animals whose sleep has been studied exhibit a sleep-rebound proportional to the amount of sleep lost, (Tobler ES 2000) [5] indicating that the amount of sleep, or of some specific intensity component of sleep reflected in ‘amount of sleep’, is physiologically obligatory. If sleep deprivation is prolonged, impaired physiological functions are manifest, ultimately causing death (as demonstrated in the rat, Rattus norvegicus) (Rechtschaffen Sleep 1989) [6].

Major human disorders of sleep manifest as changes in sleep amounts with either too little (insomnia) or too much (excessive daytime sleepiness) sleep (Carskadon ES 2006) [7]. People who experience too much or too little REM sleep, for example, tend to be at greater risk for ill-health. Increased REM sleep durations (relative to the population norm) are significantly associated with increased risk for various medical conditions and for mortality (Brabbins IJGP 1993) [8] (Dew PM 2003) [9] (Kripke PM 2003) [10] (Elgar AB 1988) [11]. The increased risk for these ‘co-morbidities’ remains even after adjusting for age, gender, mental illness and ‘medical burden’ or physical health status (Brabbins IJGP 1993) [8] (Dew PM 2003) [9] (Kripke PM 2003) [10].

Based on these considerations, the use of sleep quotas has long been a standard measure used in comparative tests, (Elgar AB 1988) [11] (Elgar AB 1990) [12] (Lesku AN 2006) [13] (Zepelin ES 1989) [14] and is the basis for construction of our database on sleep quotas.

Database Design and Construction

In order to apply the comparative method, it is first necessary to compile relevant data for as many different species as possible. To achieve this aim, we conducted an exhaustive literature search dating back to 1953, the year REM sleep was discovered. We used the key terms REM, NREM, sleep architecture, EEG and total sleep time, to search bibliographic databases that cover publications in English, French, Russian, German, and Spanish. We also identified source material from previous data compilations.

A second basic requirement for comparative analyses is that the data from different sources must be comparable. For investigations of sleep, this can be achieved by either restricting analyses to studies that applied standardized assay procedures to the relevant age or sex of animals, or alternatively, by statistically controlling for procedural differences among studies. In each of these cases, however, we require details of the methodologies that different studies employed (see below). To obtain this crucial information, we gathered data only from the original source material and recorded information on laboratory procedures.

Variation in Subject Animals

We recorded the sex of individuals tested, and separated data for male and female subjects whenever possible. Similarly, we recorded the absolute age and age class (neonate, juvenile, adult) of experimental subjects when this was provided.

Variations in Laboratory Procedures

We systematically extracted information on the following laboratory conditions and experimental procedures from the original sources:

• Recording method (EEG). Whether sleep duration estimates were based on EEG recordings or not.

• Telemetry (yes or no). Whether sleep duration estimates were based on telemetric recordings in the wild.

• Recording time as reported in hours (categorical three-state variable). We categorized all studies according to recording times in the following way: < 12 hours recording time, between 12 and <24 hours, and recording time equal to or longer than 24 hours.

• Diet (yes or no). Whether experimental animals were fed ad libitum or not.

• Normal lighting conditions (yes or no). If subjects were maintained under a ‘natural’ light condition of 12 hours light and 12 hours dark, or not (24 hours light or reversed light conditions).

• Ambient temperature (yes or no). Whether subjects were tested in an environment with temperatures within the natural range for the species under study.

• Adaptation (habituation; yes or no). Whether studies allowed the experimental animals to habituate to recording and laboratory conditions before data were collected.

• Restraint (yes or no). Whether or not subjects were restrained during sleep recording sessions.

• Behavioral observations (yes or no). Whether any sleep-related behavioral observations were noted in the species under study. This could include any mention of sleep posture, eyes opening and closing, tail jerking, penile erection during REM, neck paralysis during REM, or related behaviors.

Characteristics of Mammalian Sleep

Our exhaustive search of the literature yielded 178 studies containing data for 127 species representing 46 families across 17 orders (see Table 1).

From these data, we find that the ‘average’ mammalian adult spends 11.7 hours of each day asleep. 17% of this time is spent in REM sleep, with an average cycle length of 21 minutes between each episode of REM. A more accurate characterization of mammalian sleep may be in terms of its diversity. The amount of time devoted to sleep varies greatly, from three hours in the ass (sometimes incorrectly referenced as Equus asinus) (Ruckebusch CRSB 1963) [15] to 20 hours in the armadillo (Chaetophractus villosus) (Affani PBS 1972) [16]. The echidna (Tachyglossus aculeatus) does not appear to exhibit the classical EEG signs of REM sleep state (Allison AIB 1972) [17] (Siegel JN 1996) [18], in marked contrast to the duck-billed platypus (Ornithorhynchus anatinus), which spends 40% of its total sleeping time in REM sleep (Siegel Neuro 1999) [19]. Finally, sleep cycles may be as short as 6 minutes (in the chinchilla, Chinchilla laniger) (Van Twyver PB 1969) [20] or as long as 60 minutes (in the horse, Equus caballus) (Ruckebusch CRSB 1970) [21]. It is this remarkable variation in the nature of sleep across different species that makes the comparative analysis of mammalian sleep such an exciting and potentially fruitful endeavor.

The Special Case of Sleep in Aquatic Mammals

Members of three different orders that contain aquatic mammals – cetaceans (dolphins, porpoises, and whales), carnivores (seals, sea lions, and otters), and sirenians (manatees) have special sleep characteristics. Cetaceans and some carnivores typically engage in unihemispheric sleep (Mukhametov NL 1987) [22] (Lyamin AIB 2004) [23]. Pinnipeds within these groups are also able to sleep on land or in water, and there appears to be differences in sleeping quotas when they are asleep in water. Thus, to fully characterize sleep in these animals we require daily sleep quotas in both the right and left hemispheres on land and in the water. We have attempted to provide this information within our database, but we urge caution in using these data until a consensus emerges on how to best summarize sleep expression in marine or aquatic mammals.

How to USE the Database

The database is provided via a website funded by the National Institute of Mental Health (NIMH) (http://www.bu.edu/phylogeny/). This website provides background on hypotheses for the function of sleep, details on how the database was constructed, and a portal to search the latest version of the database. A search algorithm has been developed that allows users to search the core Phylogeny of Sleep Database (http://www.bu.edu/phpbin/sleep/search/). A glossary of search terms is provided to enable visitors to extract key information from the database. Users can perform searches of the sleep dataset according to their own criteria (for example, selecting only those studies that utilized at least 24 hour recording periods), at which point an annotated reference and selected search information will be displayed. The full reference and record for any particular species can be displayed by clicking on this annotated reference. The data from searches can be downloaded, and searches can be saved as a file with values separated by commas (i.e., a .csv file, easily read by spreadsheet and word processing programs). Additionally, the upper right-hand corner of each page provides a ‘Contact’ button that allows users to send questions and comments about the database. Finally, we have established a user-friendly web-based mechanism that allows researchers to add new data on mammalian sleep durations to the database, following confirmation of data collection procedures according to the protocols we have developed. This link can be accessed from the search database page under ‘Submit Study.’

The Need for Phylogenetic Control

Earlier studies of comparative sleep expression have called attention to the value of comparative analyses for improving our understanding of the function of sleep, most did not account for the similarity between species due to their common ancestry, an omission which can lead to erroneous conclusions (Harvey OUP 1991) [3] (Felsenstein AN 1985) [24] (Martins Evol 1991) [25] (Nunn EA 2001) [26]. Indeed, our recent analyses have detected a strong ‘phylogenetic signal’ in mammalian sleep (Capellini Evol in press) [27]. In other words, closely related mammals are likely to share common sleep characteristics, creating inter-dependencies among the data points. Thus, the use of the analytical techniques that account for “phylogenetic non-independence” are required for analyzing sleep quotas extracted from the database (Nunn EA 2001) [26].

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