As we make the transition from stage 1 to stage 2 sleep a number of changes occur in the EEG that signal the increasing depth of sleep. Two prominent markers of stage 2 sleep are K complexes and sleep spindles. K complexes are large waves that stand out from the background and often occur in response to environmental stimuli such as sounds in the bedroom. Sleep spindles are brief bursts of fast activity that appear something like the shape of an "eye" as they rapidly increase in amplitude and then rapidly decay.
Stage 2 is where we spend the majority of the night while sleeping. Typically, more time is spent in stage 2 sleep than in light sleep, deep sleep or dream sleep. It would seem that if so much time were spent in this stage of sleep that some very important function must be served. It has not been so easy, however, to discover just what this function is. In my last post, I discussed the beneficial effects of a brief nap on neurocognitive functioning. Recent research indicates that some very important brain activities are carried out when sleep spindles are occurring, whether during nighttime sleep or daytime naps.
The greatest spindle activity occurs at the beginning and the end of the non-REM portion of the sleep cycle. Sleep spindles involve activation in the brain in the areas of the thalamus, anterior cingulate and insular cortices, and the superior temporal gyri. There are also slow spindles in the range of 11 - 13 Hz that are also associated with increased activity in the superior frontal gyrus, and fast spindles in the range of 13 - 15 Hz that are associated with recruitment of sensorimotor processing cortical regions, as well as recruitment of the mesial frontal cortex and hippocampus. What do these spindles mean? Ongoing research hopes to illuminate their function.
Recent research by Matthew Walker and his research team at the University of California Berkley shows that sleep spindles are associated with refreshment of our ability to learn. This research showed that the greater the number of sleep spindles produced by napping participants, the more they were refreshed to perform on a learning task. Further, the brain areas most involved were the hippocampus and the prefrontal cortex. These are areas that are critical for learning.
The implications of this research are significant. For example, getting too little sleep and cutting off a portion of spindle activity may result in decreased ability to learn. It also has implications for the effects of early school start times on students' ability to learn at an optimal pace - a topic I have addressed in the past and will again in my next post.