The saturation rate for a healthy person while sleeping is considered to be 95% or more
It is recommended to call a doctor when the saturation drops to 94%. Saturations of 92% and below are generally considered critical. A person with such a low blood oxygen level needs urgent medical attention.
There are exceptions. For example, in severe chronic obstructive pulmonary disease, which is common in smokers, the saturation index can be from 88% to 92%.
Usually the body of such people is adapted to a lower level of oxygen. If you have a pulmonological disease, your doctor will tell you which saturation value should be an alarm for you.
If we are talking about finger devices, they are also attached to the finger, while a cord extends from them, which goes to the bracelet with the monitor. Usually, these devices are not designed for 24/7 and overnight monitoring of SpO2.