The Light, also called energy, is both the HP of the Jellyfish and its light source. It is represented as a white life bar in the top of the screen.
The Light is constantly depleting unless you are in a light zone.
In addition to the range of the light diminishing over time, when the energy goes bellow 30% the light of the player starts to flicker as a form of warning.
A full energy bar represents 100 points of energy.