SITNFlash

The basis for why we sleep is still not known, but almost everyone recognizes the need for sleep. We spend about 1/3 of our lives asleep, an average adult requiring 7-9 hours of sleep each night to feel well rested. We know that when we are sleep deprived, we are not alert and can make poor judgment calls. But is sleep really required for survival? Surprisingly, studies suggest that without sleep, animals die. For example, rats that are sleep deprived will die after 2-3 weeks, but if allowed to sleep for one full sleep cycle 2 weeks into sleep deprivation, they recover completely. Now, evidence from a growing number of laboratories suggests a hypothesis that sleep may also enhance our ability to learn and form memories. Recent work from researchers at the University of Wisconsin (Reto Huber et al.) shows that sleep helps us to learn and consolidate memories.

Reto Huber and colleagues asked subjects to do a relatively simple task using a joystick to move a cursor on the computer screen to several highlighted rotating targets. Subjects carried out the task in the evening, and the activity levels of the brain were then monitored using EEG (electroencephalogram) recording while the subjects were sleep. EEG measurements are taken non-invasively by placing a series of electrodes on the surface of the head, and the electrodes detect the electrical activity of neurons in the brain. EEG measurements can be used to monitor what brain state an individual is in. For example, an awake individual has relatively high frequency electrical oscillations, while a sleeping person has lower frequency activity. In the experiments by Huber and colleagues, the EEG recordings of the sleeping subjects showed a higher activity in the parietal cortex, a region of the brain known to be important for carrying out the chosen visuo-spatial task, than control subjects who had a modified eye-hand coordination task. In addition, after sleeping a full night, the subjects showed tremendous improvement in their ability to repeat the task the next day when compared to control subjects who did not sleep. Moreover, the higher a subject's EEG activity in the parietal cortex during the night had been, the greater the subject's improvement was on the task the next day. This work shows that increased brain activity reflected in the EEG recordings during sleep may strengthen the connection between previously co-activated brain cells and enhance learning and memory processes. While much more still needs to be done in order to understand why we sleep, researchers are finally starting to unravel some important aspects of this mysterious process!

For more information:

• http://www.sleepfoundation.org
• http://web.sfn.org/content/Publications/BrainBriefings/sleep.html
• http://www.pbs.org/livelyhood/nightshift/sleep.html

References:

• Huber, R., et al. Nature 430:78-81. (2004)
• Hairston, I. , and Knight, R. Nature 430: 27-28. (2004)
• Rechtschaffern, A., et al. Sleep 12(1): 68-87 (1989)