K-Complexes and Sleep Spindles
Spindle activity is exclusive to NREM sleep, with most occurring at the start and end of NREM. Sleep spindles engage brain activation in the superior temporal gyri, anterior cingulate, insular cortices and the thalamus. Sleep spindles have different lengths; with slow spindles associated with an increase in activity in the area known as the superior frontal gyrus ranging between 11 and 13 Hz, and fast spindles associated with the recruitment of both the hippocampus and the mesial frontal cortex and the sensorimotor processing cortical regions ranging between 13 and 15 Hz. At this point in time it’s not clear what is meant by these sleep spindles, but it’s hoped that ongoing research will reveal their function.
K-Complexes are also exclusive to NREM sleep, and can be defined as single long delta waves lasting for only a second. Like sleep spindles, they appear automatically during the early stages of sleep, generally in stage two. However, K-Complexes can be induced at will by momentary noises, such as someone knocking on a door. Further research must be conducted on K-Complexes because their function is currently unknown.
Dreaming during NREM
During REM sleep, study participants reported both intense dream vividness and improved memory of dreams which occurred during that phase, which suggests that dreaming typically occurs REM sleep: we know that dreaming also occurs during NREM sleep; however, in comparison, these dreams appear to be more mundane. We also know that dreams that occur during the NREM stage of sleep typically occur after midnight, which happens to be the time-period with the highest rate of REM sleep. This was discovered by a study whereby subjects took naps over specific periods of time and were then forcefully woken: sleep was divided into –
- naps with only REM sleep, and
- naps with only NREM sleep using polysomnography.
The implication here is that the occurrence of REM sleep is not necessary for dreaming, but rather that the actual processes creating REM sleep result in alterations to a person’s sleep experience.
By morning, and because of these changes, there occurs a sub-cortical activation that’s comparable to the one that occurs during REM sleep. Therefore, during the morning hours in the NREM stage it’s the sub-cortical activation that causes dreaming.