Suit sustains them

a unit fights in particularly inhospitable climates, they will opt for masked helmets through which nutrient gruel and oxygen can be piped. Since they rarely experience times of rest between missions, Tempestus Scions take with them the means to sustain themselves. Those who have remained plugged into their masked helmets can go for days without needing additional nutrition or sleep. They simply stand side-by-side on their transport between missions in a sort of trance, dormant yet ever alert.

Codex: Militarum Tempestus pg. 5