Interstellar journeys were very long affairs, especially in the early days; to survive decades or centuries some of the first ships were entire ecologies, with biological food production and resource recycling. Launched before and during the Great Expulsion era the few successful Generation ships ever launched allowed several generations of crew to be born, live their lives, and die en route.
However these ships were so huge that they arrived typically decades or centuries after the later, faster, smaller ships had arrived and claimed the best territory.
In order to make the journey more comfortable on the smaller ships the colonists and crew were typically placed in nanostasis, often known by the alternative name biostasis. First described by K. Eric Drexler in his classic work Engines of Creation, this technology went through many stages of development. In the pre-Federation era, this also included ultracold temperatures and tissue vitrification, a technique that eventually became widely used. Particularly in the case of poorly prepared Backyarder ships this difficult process could result in degradation of tissues on revival, including brain damage (which often responded to treatment, but also often did not).
Later nanostasis was more reliable, and often took place at normal body temperature (so called 'warm biostasis'). Body cells can be maintained essentially without change or growth, except where permitted by the controlling nanoprocessor and by dozens of tiny active nanosomes', within each cell.
It was possible for the brain tissue of the crew members to be activated for extended periods by allowing the neurons and synapses full life support and developmental freedom. Direct neural interfacing compensated for the lack of sensory input, and allowed for full consciousness to be maintained by crew members, or for extended periods of virtual reality immersion for colonists and off-duty crew.
The decades or centuries of subrelativistic flight could be filled by the construction of detailed plans for the development of habitats and energy collection arrays, and for the terraforming of any suitable worlds at the target system. These worlds could be observed in detail by the ship as it approached, and these observations added to the already detailed information available from remote observations made before the ship was even launched.
Other uses for the long periods of immobility combined with direct neural contact with the ship's multimedia library were religious contemplation and abstract thought. As a result of religious meditations on board the Ananas between Ganesha and Darwin the Bluesky sect of ecotechnicians developed their cross between practical and devotional development, resulting in the neo-buddhist paraterraforming movement; and as a result of extension of their consciousness into the technical library the crew of the Gosse developed the long distance Engenerator system.
In more recent times nanostasis has been used widely in the Civilized Galaxy to protect biological passengers on board wormhole transit ships from the disturbing effects of space time distortion, or just simple boredom. Nanostasis is also often used in high acceleration regimes, such as combat warships or stellar mining operations. Additionally many biological entities are kept for protracted periods in a conscious or unconscious state in biostasis; either while they live extended virtual lives, or while they wait for suitable environmental conditions to develop on a developing or marginal world, or sometimes simply awaiting the grand design of an inscrutable archailect.