Sleep spindles and sleep stability (JC)

This time I present you a short correspondence paper. I still need to find an efficient way to select new papers that I can discuss/summarize in a clear way, without you having to wait too long for a new paper. I’m however quite lucky when it comes to this selection, because there’s again a free version online.

Spontaneous brain rhythms predict sleep stability in the face of noise.

Thien Thanh Dang-Vu, Scott M. McKinney, Orfeu M. Buxton, Jo M. Solet, and Jeffrey M. Ellenbogen
Current Biology 2009, 20(15):626-627
Link to full online article (free)

Why do some people sleep well, while others don’t? Why do some people suffer from insomnia, while others fall asleep as soon as they hit the pillow? Why are some people easily awoken when others seem to be insensitive to even loud noises? These are some questions that come to my mind when thinking about sleep. The last question is covered in some way by the current paper.

Dang-Vu and colleagues observed that people having a higher spindle frequency (i.e. the number of spindles) during stage 2 sleep, had a higher noise threshold, i.e. a larger increase in sound volume was necessary to invoke arousal in the EEG pattern. Thus sleep stability was higher for people showing a higher spindle frequency than for people showing a lower spindle frequency. Spindles are high frequency bursts of activity (11-15Hz, i.e. 11-15 waves per second, see picture below for an example of a spindle wave) that mostly occur during stage 2 sleep, and that can be clearly distinguished from the background frequency of 4-11Hz (here frequency is the number of waves/oscillations per second).

Another interesting observation, that isn’t discussed in the paper, is the finding that the arousal thresholds for different types of sounds (e.g. traffic noise, telephone ringing…) differ. So for example at a level of 50dB people showed arousal in the case of the telephone ringing, but a higher sound level was needed to get aroused by traffic noise (hypothetical example). I find this interesting since this makes me wonder how semantics are processed during sleep.

In addition, it would be interesting to manipulate this spindle frequency and see if within a person you can change sleep stability over multiple nights. Dang-Vu and colleagues see spindle frequency as a more stable trait, but as has been shown from learning protocols, spindle frequency can increase when sleep is preceded by a learning task. In addition aging is related to a reduced spindle frequency, as mentioned by the authors. With elderly being more susceptible to sleep disturbances and showing a more fragmented sleep which is in line with Dang Vu’s findings.

Although it’s interesting to look into the function of these spindles, I’m still quite confused myself when it comes to the function. Within the field of sleep research, quite some groups now investigate different parameters of spindles and also look into how these spindle parameters change and differ across different topographical regions (e.g. parietal regions versus more frontal regions). Sleep spindles are related to cognition, sleep stability and other functions, so at this point I’m not really sure what the meaning is of this all. In addition stage 3 and stage 4 sleep are the deeper stages of sleep, consisting of little if none spindle activity, so what makes these stages the deeper stages. How come sleep stability in these stages is so high? What are the mechanisms in these deeper sleep stages? I also wonder how this spindle activity develops, what’s underlying this spindle activity in terms of neuronal activity. I have read some things about this already, but I still can’t grasp how this mechanism works and why it works the way it works. I still have a lot of learning to do, but the above article nevertheless is interesting.