From Halopedia, the Halo wiki
Bronchial surfactant is an enzyme used by humans for use in cryo chambers. The cryo-inhalant is specifically designed to coat and protect the lungs when entering cryo-sleep, then be regurgitated and swallowed upon reawakening. The clear liquid replaces some vital bodily fluids, such as nutrients lost during deep sleep. A person using bronchial surfactant is theoretically able to sleep for extremely long periods of time, thus making it possible that it includes some sort of sedative. However, the exact specifications are not known. People have been observed in cryo-sleep for up to three weeks without using the fluid.
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