Newswise — Sleep researchers at Indiana State University attracted international attention in the late 1990s with a study that found some birds can put half their brain to sleep while keeping the other half alert to watch for predators. Those same researchers are again the focus of attention with a paper summarizing decades of sleep research, at Indiana State and elsewhere, in the international research journal Animal Behaviour (October issue). "Sleep renders an animal more vulnerable to predatory attack than just about any other behavior. However, sleeping animals are not helplessly turned off, and certain states of sleep and ways to sleep are safer than others," the researchers state in the paper's conclusion. Following earlier studies involving pigeons, ducks and other birds, faculty and students have trained the 16 cameras in the Indiana State Sleep Research Laboratory on reptiles and found the practice, known as uni-hemispheric sleep, is not limited to animals with feathers. "We do think that across the animal world this is a very typical sort of thing. They all have to deal with this in one way or another, either sleeping in a very safe place so they can sleep very deeply, or if they can't sleep in a safe place they're going to have to sleep less deeply, perhaps, and monitor the environment to some extent while they're sleeping," said Steven Lima, professor of ecology and organismal biology and lead author of the paper. John Lesku, a PhD student at Indiana State, has begun studies of iguanas and initial indications are that the reptiles are capable of putting only half their brain to sleep.
"Iguanas have sort of the same evolutionary strategy as birds for maintaining some measure of vigilance concurrent with sleep," Lesku said. "Iguanas and other reptiles have all been seen for extended periods of time sitting with one eye open and one eye closed and this behavior, a very visual correlate of uni-hemispheric one-eyed sleep in birds and aquatic mammals, should also apply to reptiles. So we're presuming that they can engage in uni-hemispheric sleep and will do so more readily in environments that they perceive to be riskier."
Humans, who generally feel safe and secure when sleeping in their own beds, do not show any indication of being able to shut only half their brains down at a time, but that is not to say that external factors do not influence human sleep, Lesku noted.
"Studies have shown that humans sleeping in more stressful environments, or more unknown environments such as a hotel room, will have more fragmented or poorer quality of sleep. Also there are certain behavioral symptoms that people can be afflicted with, such as anxiety or depression, that also alter sleep architecture in a way that is consistent with animals sleeping under the risk of predation. The mechanism of this may be simply higher levels of circulating stress hormones or something like that which are suppressing these deeper stages of sleep." Despite a half century of concerted research on humans and decades of research on a broad spectrum of animals, "we don't know the absolute answer as to why one sleeps," said Charles Amlaner, professor and chair of Indiana State's department of ecology and organismal biology. "What we do know is that there are a number of factors out there that will shape sleep. Sleep is not something that is just so locked in that it won't adjust based upon external pressures." Lesku, Amlaner, and Niels Rattenborg, who completed his PhD at Indiana State and is now at the University of Wisconsin at Madison, co-authored the paper with Lima.
While some researchers, including those at Indiana State, conduct studies in the field, most work is done in the more controlled setting of a laboratory "that may not capture all of the salient features of sleep," the paper says.
"In the study of sleep, we have lacked a sort of an evolutionary perspective on why we sleep and what some of the functions might be," Lima said. "We need both approaches " a more mechanistic sort of clinical approach and an evolutionary approach to really fully understand this whole phenomenon of sleep." That full understanding won't come, researchers say, until they answer the question of just why we sleep. "Wouldn't this be one of the cruelest hoaxes of natural selection and the process of evolution? If sleep really isn't an adaptive behavior, why are we doing it? With animals and man sleeping at such a high level, we've got to believe that there's a function out there and we're going to find it one day," said Amlaner.
RSS