In literature (by neurotypical people) stimming is mostly characterized as:
Visual
staring at lights, repetitive blinking, moving fingers in front of the eyes, hand-flapping
Auditory
tapping ears, snapping fingers, making vocal sounds
Tactile
rubbing the skin with one's hands or with another object, scratching
Vestibular (sense of balance)
rocking front to back, rocking side-to-side
Taste
placing body parts or objects in one's mouth, licking objects
Smell
smelling objects, sniffing people
But in my opinion this is merely the physical outcome from what is happening inside.
So I would count daydreaming as stimming as well as having repetitive music in your head.
Quote:
Why does stimming, or self-stimulation happen?
Researchers have suggested various reasons for why a person may engage in stereotypic behaviors. One set of theories suggests that these behaviors provide the person with sensory stimulation (i.e., the person's sense is hyposensitive). Due to some dysfunctional system in the brain or periphery, the body craves stimulation; and thus, the person engages in these behaviors to excite or arouse the nervous system. One specific theory states that these behaviors release beta-endorphins in the body (endogeneous opiate-like substances) and provides the person with some form of internal pleasure.
Another set of theories states that these behaviors are exhibited to calm a person (i.e., the person's sense is hypersensitive). That is, the environment is too stimulating and the person is in a state of sensory-overload. As a result, the individual engages in these behaviors to block-out the over-stimulating environment; and his/her attention becomes focused inwardly.
Researchers have also shown that stereotypic behaviors interfere with attention and learning. Interestingly, these behaviors are often effective positive reinforcers if a person is allowed to engage in these behaviors after completing a task.
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English is not my native language, so I will very likely do mistakes in writing or understanding. My edits are due to corrections of mistakes, which I sometimes recognize just after submitting a text.