When the Borg had assimilated individuals who were of a a young age, they were placed in a maturation chamber until they reached the age of a thirteen year old human, or an equivalent developmental stage of other species. (VOY: "Collective") From that age on a drone would make use of an alcove which was specifically attuned for the individual drone, although minor adjustments could be made to accommodate use by other drones.(TNG: "Q Who") The drone would then return to the alcove on a semi-regular basis for the purpose of regenerating mental, physical and electromechanical energies spent through normal activity.
Alcove Design and Technology
All alcoves were relatively the same in basic design with a few exceptions. One such exception was the Borg Queen – she had a different alcove, called the central alcove. Another notable alcove which featured a unique design was a portable alcove designed by Seven of Nine. Although it is unclear if this alcove was unique, it featured the same purpose as a "normal" alcove in a compact and portable design. It was used by adapting a Neural transceiver to interface with the portable alcove. (VOY: "Dark Frontier"); (VOY: "Child's Play")
An alcove was sized to allow the drone to fit inside. It was sized slightly wider and higher than a drone, with a small raised platform for the drone to stand upon. A green disc large enough to accommodate the drone's head was positioned at the height of the drone. Unnecessary to its use, it pulsated visually with various patterns, some static and others dynamic, usually green and white in color. (VOY: "Dark Frontier")
Once positioned into the alcove, a drone's mechanical parts established a connection with the alcove's systems (much the same way a doctor would monitor a patient with a biobed. The alcove then established a connection in one of two ways: either through a port resembling an arm rest onto which a drone would connect its arm, or by means of tubes which connected directly from the wall into various ports in the drone exo-plating (TNG: "Q Who"; VOY: "Scorpion").
When the Borg drone stepped into the alcove it would stand upright, if its species physiology permitted. During the entire regeneration cycle a drone would have to face outwards within the alcove, although some alcoves allowed drones to face either outward or inward. (VOY: "Drone", "Q Who")
When a drone's cortical node malfunctioned a regeneration cycle could not be started. If a drone remained in an unregenerate state for too long, it lost motor control, and eventually became unconscious; a state very undesirable to the Borg. If a Borg drone was unable to continue to fully regenerate as it needed (due to injury or malfunction), it would be removed from its alcove by other drones and its components were salvaged for reuse.(VOY: "Dark Frontier")
With knowledge of Borg technology, alcoves could be integrated directly into Starfleet vessels to serve the same purpose of their counterparts on Borg vessels. When this was done they would require over thirty megawatts of power to run each alcove. (VOY: "Dark Frontier") Or if needed, a Borg drone could regenerate by means of a converted Federation power conduit which would function similar to an alcove. (TNG: "I Borg")
Placement Within the Vessel
Most Borg vessels situated alcoves along corridors or other "wasted" space within a Borg vessel. Depending on the type of vessel there would be at least as many alcoves as there were drones. When an alien ship was captured and assimilated, the alcoves were created where there was space to place them. (Star Trek: First Contact) An alcove was a separate unit that could be beamed to another ship when the situation required it. Alcoves could be placed separately or clustered together with a Borg computer console at the end. (VOY: "Scorpion, Part II")
Alcove Regenerative State
A Borg drone would enter its regenerative state similar to sleep for a period of time called a regeneration cycle. A regeneration cycle would vary in length based on factors such as: how long it had been since the drone last entered an alcove; what species and age the drone was; was the drone injured and was the drone needed for assignment or task. A typical regeneration cycle took a full 6 hours to complete. (VOY: "Child's Play") When part of the collective a drone could go without regeneration for roughly two hundred hours before it suffered serious ill-effects. (VOY: "Hunters")
Although during regeneration a drone would enter a mental state similar to that of sleep, it remained far from unconsciousness. Drones who were disconnected from the Borg Collective and hive mind were able to stop the regeneration cycle themselves at their desire, although an incomplete regeneration cycle is likened to being woke from slumber and can have an effect of grogginess. (VOY: "Child's Play")
A drone that was functioning normally and part of the Hive Mind did not dream, instead a drone's mental capacity was apportioned to various tasks. These tasks sometimes involved the control of nanites. For instance, a drone may be utilized to repair damage to a vessel or to heal injured comrades while remaining semi-conscious in the alcove. In some situations a drone would preform various tasks that were directly connected to other drones or the ships systems while remaining connected to the alcove. (TNG: "Q Who"; VOY: "Unity")
A regeneration cycle could be interrupted without the drone ever knowing using various technology that prevented the drone from "waking" when the cycle was interrupted. The Hansens used this technique to examine Borg drones by beaming a drone from its alcove to their ship.
When drones were in their alcoves Federation scanners seem to detect only a single life sign when scanning a Borg vessel. This was partly due to the nature of the Hive mind and partly due to the nature of Borg cybernetics which integrated most bio-electromechanical functions into the Borg systems.(TNG: "Q Who")
In 2377, the Borg discovered that drones with a genetic mutation used their alcoves to create Unimatrix Zero, in which they could interact which each other while regenerating, although the memories and experiences gained within it would not stay with the drone when the regeneration cycle ended. (VOY: "Unimatrix Zero", "Unimatrix Zero, Part II")