From Gineipaedia, the Legend of Galactic Heroes wiki
Tank Beds are vertical chambers, each large enough to contain a single adult human, designed to decrease fatigue in combat situations.
A single hour inside a tank bed is said to be just as refreshing as a full 8 hours of sleep. Tank beds are not, however, considered the equal of actual sleep (soldiers are still assigned regular quarters with cots or beds depending on rank) and are therefore only used during periods of combat or combat-alert status.
A common technology in both the Galactic Empire and Free Planets Alliance, these beds were indispensable as large-scale fleet engagements could often go on for hours, days or even weeks of near-constant combat.