Analysis of electroencephalographic rhythms helps uncover a little of the mystery of what goes on while we sleep. If you place electrodes on the scalp of a sleeping person, you can see patterns of rhythmic discharges of neurons through the thalamocortical system.
Sleep spindles are squiggles that show up in the EEG readings of Stage 2 sleep. The EEG shows voltage difference fluctuations, and the lumping together in a close time of a flurry of fluctuations suggests something going on in the brain like a transfer of electrical energy. They’re called spindles because of how they look when printed out.
The frequency of spindles seems to indicate how active the brain is and serves as a physiological index of intelligence. The frequency also increases during the night following when a person has learned something new. Further, when there are more spindles, the person’s performance at a recently learned task or skill increases the next day. This is why scientists are convinced spindles indicate a transfer of memory from short term (daily) to long term memory. How memory is stored in the brain is still a great mystery, but at least we can see indication of long-term memories forming on an EEG.
Spindles may also find use as a biomarker for schizophrenia some day.
Tests with animals have shown the spindles happen during electrical energy transfer between the hippocampus and neocortex.
In the brain the thalamic and corticothalamic networks are involved with the electrical activity that produce a spindle on an EEG. Physiologists are trying to distinguish among different types of spindles. Some are fast (13–15 Hz) spindles in the centroparietal part of the bran while the frontal brain produces slow (11–13 Hz).
Melatonin seems to promote spindles. Spindles indicate a transfer of information between the hippocampus and the neocortiex. The electrical activity that gives rise to spindles is both global (over much of the brain) and local (only in parts of the brain).
When the person learns something new the number and duration of spindles in the next sleep period increases. Both the density (number in a given perod of time) and sigma (14-16Hz) spectral power go up after a nap as performance and vigilance measures go up. http://www.ncbi.nlm.nih.gov/pubmed/21167865?dopt=Abstract
Afternoon naps take the sleeper into Stage 2, and spindles can be observed. In young children who are maturing and learning, the spindles show up in analysis of their afternoon naps.