These rythhmich bursts are refered to as sleep spindles.
Sleep spindles are precipitous bursts of oscillatory activity (neural activity) in the brain, which in turn are reffered to as rythimic bursts because they follow certain patterns.
As for the name "spindles" it comes from the EEG reading of the activty, where you can visualize areas with what appear to be spindles.
Hope it helped,
BioTeacher101