What does sleep efficiency mean?

If you wear your Armband while you sleep, it calculates "sleep efficiency." This is the ratio between sleep time to lying down time. Adult efficiency levels are typically in the 80% to 95% range, and decrease with age.
Why am I seeing gaps in my sleep graph or steps recorded during sleep?
Occasionally gaps or steps may be recorded during sleep depending on how you are sleeping during that particular night. It could be that you are actually awake, have some abnormal arm movement or position, or the band was moved during sleep so the sensors lose contact with the skin.