A Toa's heartlight flashed rapidly when he ran.
A Matoran's heartlight could fade to black (presumably indicating something bad).
- BIONICLE Chronicles #4: Tales of the Masks, p. 40 (on Takua as a Matoran)
- BIONICLE Adventures #3: The Darkness Below, p. 91 (on a Toa Metru)
"The Matoran within slept an unnatural sleep, eyes dark, heartlight faintly pulsating.
"Axonn was on top of the Toa in the flash of a heartlight, axe blade at the intruder's throat."
See The TTV Message Boards [source].