Cover Story

Studies of sleeping animals are gleaning new insights that could help psychologists understand human sleep and develop effective treatments. Below are a few of the most promising findings.

Sleeping with one eye open

Imagine being able to sleep with one eye open--and half your brain awake. Recent research confirms that dolphins and whales do--a phenomenon known as unihemispheric sleep.

Researchers theorize that some animals have developed this ability to help them detect approaching predators or to periodically rise to the surface to breathe and monitor wave patterns, which keeps them from drowning.

While humans do not practice unihemispheric sleep, it is conceivable that particular regions of the human brain stay awake while others sleep, or that the intensity of sleep may vary across different brain regions, says Niels C. Rattenborg, PhD, of the University of Wisconsin, who has studied a similar phenomenon in birds.

The finding is important, he says, because it shows that at least in some animals, "sleep and wakefulness can be independently controlled within different regions of the brain according to changing ecological demands."

A genetic root for narcolepsy

Studies with mice and dogs have shed light on the cause of narcolepsy--the sleeping disorder characterized by sleepiness, muscle weakness and abnormal states of rapid eye movement (REM)--affecting more than one in every 2,000 Americans.

In 1999, researchers at Stanford University and the University of Texas Southwestern Medical Center separately reported that they had uncovered a genetic mutation that causes narcolepsy in dogs and mice, respectively. Soon after, Stanford's Emmanuel Mignot, MD, PhD, found that dogs that had the mutation also had a deficiency in a receptor for hypocretin, the neurotransmitter that blocks communication between brain cells, particularly messages about when the body should be awake.

These findings led sleep researchers to suspect the disorder might have similar roots in humans. That theory was confirmed in 2000 by both Mignot and Jerome Siegel, PhD, of the Sepulveda VA Medical Center and the University of California, Los Angeles. Since then, Siegel has discovered that narcolepsy symptoms could be reversed in dogs by injecting hypocretin, providing the first indication that the neurotransmitter might be key to treating the disorder in humans. Complicating that effort, however, is the fact that in humans narcolepsy is caused by a combination of genes as well as by environmental triggers.

Siegel and his colleagues are continuing to study how hypocretin affects narcolepsy in humans and what unknown long-term physical damage the disorder causes. They also hope to begin to develop drug treatments to combat narcolepsy.

Fear conditioning

Although sleep researchers have probed many regions of the brain to understand the neurological underpinnings of sleep, the amygdala has been woefully overlooked, says University of Pennsylvania sleep researcher Adrian Morrison, DVM, PhD. In a new study, published in the April issue of Archives of Italian Biology (Vol. 139, No. 3), Morrison and colleagues Larry Sanford, PhD, of Eastern Virginia Medical School and Richard Ross, MD, PhD, of the University of Pennsylvania examined the amygdala's role in insomnia, specifically looking at the effect of fear conditioning on sleep and wakefulness in rats.

Morrison's research team administered mild electrical stimulation to rats' footpads paired with a conditioning stimulus to elicit a fear response in the amygdala--a procedure designed to mimic brain activity in human anxiety disorders. In this way, the researchers could trace the amygdala's role in alertness and arousal during sleep.

In rats that were subjected to the fear conditioning, the researchers detected suppressed REM sleep during subsequent sleep cycles. That pattern did not occur when no fear stimulus had been presented. Thus, the researchers concluded, fear-inducing stimuli suppress the brain activity that should happen during REM and the consequent immobilization, and confirmed that the amygdala's activity can affect sleep patterns.

These findings have spurred Morrison to conduct more research about the amygdala's role in insomnia, particularly the types of amygdalar transmitters involved in fear conditioning's effects on sleep.