New research reveals competition between brain hemispheres during sleep

Human beings are bilaterally symmetrical. As such, our brains are made of two halves called hemispheres, which communicate with each other using specialized fiber tracts running across the midline. While each hemisphere tends to deal with the senses (vision, hearing, touch) and motor control of the opposite side of the body, we are generally not aware of this partitioning of function, thanks to constant inter-hemispheric communication. In humans, the two hemispheres are also specialized for certain functions: language areas, for example, are typically in the left hemisphere.

Luis Riquelme and Gilles Laurent of the Max Planck Institute for Brain Research in Frankfurt, Germany report in Nature that during one phase of sleep, the two halves of the Pogona brain compete with one another such that one side imposes its activity on the other, until the dominant hemisphere switches over to the other side, alternating back and forth throughout the night.

Lorenz Fenk explains, "Sleep in Pogona is divided into two states, similar to those described in mammals, including humans: a phase of so-called slow-wave sleep, where the electroencephalogram shows low-frequency waves—hence the name—and a second phase, called REM (for Rapid Eye Movement) or paradoxical sleep, where the EEG resembles that recorded during the awake state (hence 'paradoxical') and the eyes tend to make jerky movements under the eye lids (hence REM) while the body is otherwise paralyzed."

In humans, sleep starts with a long slow-wave phase (for about 60 minutes) followed by 5-10 minutes of REM, and this alternating cycle starts over again, 5-7 times per night. As the night progresses, the fraction of REM sleep increases at each sleep cycle. In Pogona, the sleep cycle is much shorter (less than 2 minutes) and the two sleep states are equal in duration (45-60 seconds each) throughout the night. A dragon undergoes 250-350 such sleep cycles each night, alternating regularly between its versions of slow-wave and REM sleep.

By recording neuronal activity simultaneously from the same area (called the claustrum) on the two sides of the Pogona brain, the scientists discovered that each side operates independently of the other during the slow-wave phase of sleep. To their surprise, however, the two sides became precisely synchronized during REM, but with a very short delay of 20 milliseconds (a millisecond is a thousandth of a second) between the left and right brains. More surprising still, they found that the side leading the other by 20ms switched on average once per sleep cycle between left and right sides.

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