While We Were Sleeping.

SOURCE:  http://gizmodo.com/371995/sewer-gas-may-allow-for-suspended-animation

Sewer Gas May Allow For Suspended Animation

Suspended animation — the state that space travelers are always put into in movies so they don't age while traveling for many years — may turn out to actually be possible, and it's a simpler process than you'd expect. Yep, all you need to fall into a deep sleep and not age is a good dose of sewer gas.

Scientists have discovered that small doses of hydrogen sulfide put mice into a completely reversible state of metabolic suppression. Within minutes of inhaling the gas the mice began to show the effects and were able to snap out of it within 30 minutes of the air supply being returned to normal.

If this can be done to humans, it could be used to "allow organ function to be preserved when oxygen supply is limited, such as after a traumatic injury" as well as, you know, for long-term space travel.