How does ZeFit calculate my sleep quality? December 08, 2014 15:35 Our 3 axis accelerometer sensor determines whether your are in a phase of light sleep, deep sleep, or awaken. Related articles How does ZeFit calculate my steps, distance and calories burned? How do I manually activate sleep mode? What does ZeFit track? My ZeFit shows FULL MEMORY and no longer tracks my activity ... How long does the battery last after a full charge?