Birth by Sleep is an action roleplaying game that introduces a gameplay aspect called the Command System, which allows players to customize a Command Deck with techniques and abilities that they can perform at will, called Deck Commands. Performing ordinary attacks and Commands fills a gauge displayed above the command deck that, when certain requirements are fulfilled, activates a Command Style that changes the player's basic attack command with the "Surge" command, which racks up powerful combos depending on what Command Style is activated. Filling the gauge with certain requirements a second time causes the character to change into an even higher level Command Style, which replaces the "Surge" command with the more powerful "Storm" command.