Technology to induce a state of deep hibernation in human beings was developed early in the course of Imperial history, having long been considered a practical necessity for long voyages. Hibernation allows people to rest, unconscious, for weeks or months at a time, with their needs for nutrition, oxygen, and hydration dramatically reduced. Body temperature decreases markedly. Hibernation also slows the ageing process tremendously, with persons spending many cumulative years in hibernation appearing much more youthful than their chronological age would suggest.
Induction
For elective hibernation (i.e. not as part of emergency therapy) the person entering hibernation undergoes a pre-hibernation health check, and takes a cocktail of oral medications 24 hours in advance of hibernation, during which the person must also fast. These medications will tend to make the person very tired within a few hours of being taken, and he or she will usually go to sleep without effort. The hibernation technologist, or a medical professional specializing in hibernation or spaceflight medicine, makes some final physiological checks (and examines lab results from the pre-hibernation exam) and connects an specialized anti-infective long-term IV line to supply required nutrients and maintenance dosages of hibernation drugs for the duration.
The most popular metabolic modulators for hibernation are the drugs somnohyrin and resohyrin, or combinations thereof. Neither is very expensive.
Maintenance
Although unusual, it is possible to survive in hibernation for decades with occasional waking. However, hibernation duration is usually limited by caution and social, rather than biological, considerations. A hibernation technologist attends regularly to hibernating persons, ensuring that their physiological parameters are acceptable, they remain free of infection, etc, and take appropriate action (adjusting the hibernation parameters or waking the person) as necessary. Hibernating persons lie on their backs on vibrating, pneumatic mats that prevent bedsores and reduce bone loss. Some of the hibernation cocktail drugs do not have to do with maintaining the actual metabolic state of hibernation, but are to prevent side effects such as muscle atrophy.
Cessation
Minimally, removing the hibernation IV and allowing time to pass will usually bring hibernation to a safe if not ideal conclusion. (The person will be very thirsty and groggy when he or she wakes up after such an abrupt cessation.) However, it is usual to swap the IV for a "wakeup" cocktail supplying continued hydration and some of the drugs in the maintenance cocktail, but discontinuing the metabolic modulators that actually cause hibernation. Regular procedures allow the hibernating person to wake up naturally as if from ordinary sleep, with their usual mental faculties. The ideal procedure takes about a day, with the crewmember being fully ready to resume duties 20-30 hours after the discontinuation of the metabolic modulator drugs. Minor digestive difficulties relating the the reestablishment of normal gut flora activity are the most common issue, and are usually treated prophylacticly with oral medication once the person wakes up.
"Crash" wakeups are possible in an emergency (e.g. for evacuation) and on some ships, a syringe with crash wakeup drugs is stored with every person in the hibernaculum, to be administered in case the ship must be abandoned or hiberating crew must be moved under their own power. Crash wakeup is unpleasant and is avoided. About fifteen minutes after the drugs are administered, the person will be able to move with assistance and follow simple verbal instructions.
Safety
As practised by modern space travellers, hibernation is safe and effective, with a low rate of complications and side effects. (The chance of a serious hibernation complication is less than one per million inductions, comparable to a flu shot.) Statistically speaking, hibernating crewmembers are also safer than the awake minimal crew on long voyages. The awake crew includes one or more hibernation medical technician who checks up on each hibernating person daily for the duration of the voyage, assisted by medical telemetry. Pre-hibernation physical exams are an important aspect of a hibernation safety protocol.
Hibernation is absolutely contraindicated in fungal and certain other infections, and relatively contraindicated in infections generally and trauma, as it slows the wound-healing process and immune system activity. Certain tumors may be less responsive to the induction of hibernation than healthy tissues, though this is the exception rather than the rule. Conversely, hibernation, especially histoselective hibernation which differentially slows the metabolic activity of some tissues over others, has found use as a palliative cancer therapy or stopgap measure until definitive therapy can take place. Crash induction of hibernation is used under carefully controlled circumstances in the treatment of certain severe viral infections, and to ease the strain on the cardiovascular system in some disease states.
Facilities
"Sleeper ships" for mass passenger transport have specialized hibernacula in which hibernating people are stored very closely packed, having been induced before being brought onboard already hibernating. In contrast, for ships where all the crew are required to be simultaneously active in some situations, the normal accommodations are also used for hibernation, and they often have adaptations to this secondary use (e.g. medical telemetry links in crew sleeping spaces.)
Social Aspects
Hibernation is a great boon to long-duration space travel in that users are spared tedious weeks of confinement to the ship with little to do. (Spacecraft are highly automated and can be supervised with small skeleton crews if they aren't executing non-routine operations.) What is more, they also age little during the time spent in hibernation. However, it is not a perfect solution. Time spent in hibernation rolls past at its usual pace for family members and friends back home. Most spacing organizations impose some kind of a lifetime limit on hibernation time, offering early retirement if it must be exceeded. For similar reasons, most Space Forces base enlistment obligations (and pay) on chronological time, not awoken time. Some spacers actually seek out work that will require extensive hibernation, so as to get to retirement quickly (in subjective time.)
Long Term Biological Effects
Hibernation has long-term biological effects, some of which are considered desirable. Most notably, the process of ageing is markedly reduced during hibernation. As a rule of thumb, forty days in hibernation are equivalent to one day of normal activity. Different ageing processes are affected to differing degrees by cumulative hibernation. People who have accumulated many years of hibernation show a different distribution of ageing-related illnesses and physiological changes than non-hibernating populations, but since most habitual hibernation-users are also frequent space travellers, the exact effects have been difficult to disentangle from the effects of the spacecraft environment. In any case, it is clear that hibernation markedly extends chronological lifespan and delays the onset of ageing-related debility.
Pregnancy
Hibernation prolongs pregnancy. It his considered safe in the early stages of pregnancy, but data is deficient and there are concerns about its effects on a developing fetus. Thus, elective hibernation is considered contraindicated in pregnancy, but pregnancy tests are not normally required as part of pre-hibernation exams.
In Children
Hibernation has complex effects on child development. Therapeutic hibernation is used in pediatrics, but elective hibernation is generally considered contraindicated in children. (And, controversially, by a minority of hibernologists in young adults.) The Allied Academy of Hibernology's evidence-based guidelines recommend 16 Earth years as the minimum age for elective hibernation in space travellers.
Other Suspended Animation Technology
Cold sleep has also been successfully developed. During cold sleep, the body does not age at all in the conventional sense, and microorganisms are also in suspended animation. It is suitable for cases of infection and trauma where hibernation would be impossible. However, the induction of cold sleep and restoration from it are exceedingly dangerous by comparison. Because the population treated with cold sleep is considerably less healthy than the largely elective population seen with hibernation, statistical data is not as strong, but the odds of a serious complication of cold sleep is something like one in a hundred.
Persons in cold sleep are prone to cumulative genetic damage, where background radiation and natural radioactive elements within the body (e.g. potassium 40) damage DNA. The body ordinarily repairs such damage (including when in hibernation, albeit at a reduced rate) but cannot do so in cold sleep. This limits the maximum possible duration of cold sleep, though it can be extended with periodic intervals of waking. Such wake-cold sleep cycles, though, increase the cumulative risk of an induction or cessation complication.