We should stop feeling guilty about taking that "power nap" at work or catching those extra winks the night before our piano recital, according to new research (Nature Neuroscience 2002;5:618-9, 677-81, July).
Sara Mednick and colleagues show that "burnout"--irritation, frustration, and poorer performance on a mental task--sets in as a day of training wears on. Subjects performed a visual task, reporting the horizontal or vertical orientation of three diagonal bars against a background of horizontal bars in the lower left corner of a computer screen. Their scores on the task worsened over the course of four daily practice sessions. Allowing subjects a 30 minute nap after the second session prevented any further deterioration, and a 1 hour nap actually boosted performance in the third and fourth sessions back to morning levels.
Rather than generalised fatigue, the researchers suspected that the burnout was limited to just the brain visual system circuits involved in the task. To find out, they engaged a fresh set of neural circuitry by switching the location of the task to the lower right corner of the computer screen for just the fourth practice session. As predicted, subjects experienced no burnout and performed about as well as they did in the first session--or after a short nap.
This led the researchers to propose that neural networks in the visual cortex gradually become saturated with information through repeated testing, preventing further perceptual processing. They think burnout may be the brain's mechanism for preserving information that has been processed but not yet been consolidated into memory by sleep.
Recordings of brain and ocular electrical activity monitored while napping showed that the longer 1 hour naps contained more than four times as much deep, or slow wave, sleep and rapid eye movement (REM) sleep than the half hour naps. Since a nap hardly allows enough time for the latter REM sleep effect to develop (it usually only happens in the late stages of sleep, early in the morning), a slow wave sleep effect seems to be the antidote to burnout.
Neural networks involved in the task are refreshed by mechanisms of cortical plasticity operating during slow wave sleep, say the researchers. "Slow wave sleep serves as the initial processing stage of experience dependent, long term learning and as the critical stage for restoring perceptual performance."
The new findings have implications for learning sports or a musical instrument, or developing artistic movement control. "All such learning of new actions may require sleep before the maximum benefit of practice is expressed," note the researchers. Since a full night's sleep is a prerequisite to experiencing the critical final two hours of slow wave sleep, erosion of sleep time could short change people's brains of some learning potential.
