Cryonics

Cryonics is the science of preserving a human or other organism in stasis for extended periods of time.
The goal of cryonics is to allow an individual to "sleep" through long intervals of time, without suffering the effects of biological aging, and then allow them to wake up again without ill effects. Cryonic suspension can reduce the human body's metabolism to near zero, thus preventing aging and eliminating the body's need for oxygen, food, and warmth. The UNSC uses cryonic storage pods in most warships, maintaining only skeleton crews or computer/AI control until the vessel reaches their target zone. The majority of personnel are placed in cryonic suspension for any voyage over 120 hours to prevent aging during the journey and to conserve onboard resources.