Forgotten how to do something you just learned yesterday? Consider the possibility that last night's sleep was punctuated by mini-awakenings, robbing you of the ability to commit that new skill to memory. You might have gotten eight hours of sleep, and may not even feel tired. But when sleep is interrupted frequently--as it is in a wide range of disorders, including sleep apnea, alcoholism and Alzheimer's disease--the ability to learn new things can be dramatically impaired, says a new study conducted on mice.
The research, published this week in the Proceedings of the National Academy of Sciences, used a novel method to isolate the effects of sleep fragmentation from overall sleep quality. Studies to date have shown that when sleep is frequently interrupted, memory suffers. But no one really knew whether the memory problems they observed were the result of shorter cumulative sleep times, poor overall sleep quality, the degradation of some distinct part of the sleep cycle, or the sheer annoyance of being prodded awake repeatedly while sleeping. This study suggests that even when frequent waking doesn't affect sleep quality and doesn't cut into overall sleep time, memory takes a hit.
Researchers at Stanford University stimulated "microarousal events" in mice by injecting their brains with a virus carrying a red fluorescent protein. Once established in the brain, the protein found its way to specialized brain cells in the hypothalamus involved in awakening. When stimulated by a laser diode directed at that region of the brain, those specialized neurons became active and the mice briefly awakened. During four hours of daytime sleep, scientists "lit up" the awakening neurons every 60 seconds, causing the mouse's brain briefly to stir, and then fall back to sleep. The frequent awakenings did not drive down the amount of rapid-eye-movement (REM), or deep, restorative sleep the mouse had. Nor did it drive down cumulative sleep time. And it didn't appear to cause the mouse any stress.