Some models of the human mind see consciousness as only the topmost layer of many mental processes, most of them unconscious, that form the mind of the person as a whole. During the First Federation period experiments found that it was possible to create a mind structure which, while still able to function fully as a human being, was at the same time either more or less conscious than an unmodified baseline human, with more and less of the mind's functioning below the threshold of consciousness.
There was no real advantage (for baseline minds at least) in having an increased consciousness that was constantly present; it was found to be so distracting that it basically prevented them from doing anything else, and so permanently increased consciousness tends to found only in transapient entities. However, being able to be conscious of and consciously control such things as eye dilation, blood pressure, pulse, cholesterol uptake, cortisol levels, serotonin levels, endorphin levels, pain response and voice stress patterns when required was found to be useful, although some of these could be dangerous to the user.
The modification where only the minimal conscious activity occurred was considered useful as a means of social control and, when switch-able, for use by individuals engaged in boring or otherwise unpleasant activities, as something analogous to an improved version of Soma.
As such, because of the way it worked, people with the 'minimal consciousness' modification became known as 'Sleepwalkers'. They can function and interact as well as a normal baseline human but most of what they do happens unconsciously, and they are only aware and 'awake' when something comes up that has to be handled by their conscious mind. As such they generally do not remember what has happened the rest of the time.
The Sleepwalker modification, especially the switch-able kind, has spread far and wide across Terragen space and is now found in many clades and types of sophonts.
The Sleepwalking state does not, however, count as real sleep. In the Sleepwalking state most of the brain is still functioning as it would if the person was 'fully' conscious, and so proper sleep is also required for health.
In some less ethical polities the Sleepwalker modification is externally controllable and is used as a means of social control by the rulers of the polity. Sometimes used simply for control, in other places it is implemented as a genuinely well-intentioned means of keeping the general populace happy while also providing a solution to the lack of useful baseline professions in Terragens space. In these cases it allows people to perform what, to them, appears to be meaningful work while in an unconscious state, even though the work is actually utterly meaningless . They shuffle paper, receive their stipend, and go home like good little wage slaves, never realising that it is (for example) the same ten papers being passed around from day to day.
Some unethical polities also provide a strong psychologically addictive effect to the 'Sleepwalking' feeling, so that people want to go to work. In many places where this is done work is (perhaps 'for technical reasons') the only place where people can trigger the Sleepwalking effect.
In some polities Sleepwalking is used to cover both work and education. People are entirely capable of learning while in the Sleepwalking state, but are much easier to control while doing so. Of course precisely what is taught while in the Sleepwalking state varies. In some places a 'proper' education is provided. In others merely enough teaching to perform a set task, with the rest being indoctrination. Some of these latter polities also ensure that the Sleepwalker-trained knowledge is only accessible while in the Sleepwalking state.
Thus although it can have its advantages, the 'Sleepwalker' modification is widely considered to be one of the more unpleasant iron-fist-in-a-velvet-glove sophont-controlling technologies that pervade Terragens space.