Cold sleep was a medically-induced state of suspended animation used for interstellar travelers. It allowed them to travel between stars for many years without aging during their journey. It was also sometimes used to keep older individuals alive until an anticipated future event in which their talents would be needed.
The medicines used to induce cold sleep are species-specific: The Race, which developed the practice, also created the drugs used to put Rabotevs and Hallessi into cold sleep. Humans developed their own drugs in the late 1970s. People were put into cold sleep for a variety of reasons, and came to be known as Rip Van Winkles, after Washington Irving's classic tale.
Within the first two decades of the invention of human drugs, rapid progress was made in the effectiveness and especially in the safety of the drugs. Nevertheless, they were never perfected, and several human patients failed to recover, including The Doctor who had been chosen as America's first ambassador to Home. In contrasting this to their own cautious centuries-long development of the other three versions of cold sleep, the Race considered this margin of error an unacceptable risk. Nevertheless, they agreed to use these drugs on Kassquit, a Tosevite citizen of the Empire, when she asked to allowed to travel Home. Being put under the human drugs was an experience akin to a continued slowing of the mental faculties. When recovery was successful, the patient was disoriented on emerging. Human medics made these patients unfasten straps on their own gurneys before getting up as a sign that they had regained function.
In every species, spending much time in cold sleep made an individual feel like an anachronism, as friends and loved ones grew old and often died while the cold sleeper was in suspended animation. There were also noticeable shifts in social trends that left the travelers feeling behind the curve, even in the slow-moving society of the Race.
Among humans, cold sleep became obsolete when the United States developed FTL technology, allowing interstellar travel to take place within a period of weeks. As the Race began pursuing its own FTL technology when the Commodore Perry arrived at Home, it was speculated that the same would soon be true for the other three species.