The book 'The Future Happens Twice' explores the scientific progress expected to be made in the next 50 years. Cryonics is the practice of cryopreserving humans that can no longer be sustained by contemporary medicine until resuscitation may be possible in the future. A sleeper ship is a hypothetical type of manned spaceship in which most of the crew spends the journey in some form of hibernation or suspended animation. As there is currently no known technology that allows for long-term suspended animation of humans, the term is only found in science fiction. One alternative may be cryogenic freezing of the crew, though that would not be true suspended animation. The most common role of sleeper ships in fiction is for interstellar travel, at slower-than-light speed. Travel times for such journeys could reach into the hundreds or thousands of years, making some form of life extension such as suspended animation necessary for the original crew to live to see their destination.