This is not a thermometer. A digital sensor sitting against the inside of your finger tracks the temperature at the surface of the skin through the night. On its own that figure means very little. What carries information is the distance between tonight and your own nightly average.
The rhythm underneath
Body temperature is not constant. It follows one of the clearest circadian rhythms in human physiology: falling through the evening as the body prepares for sleep, reaching its lowest point in the early hours, then climbing again in the hours before you wake.
That evening fall is not a side effect of sleep — it is part of how sleep is initiated. A night where the pattern flattens or arrives late is a night that behaved differently, whatever the clock says about how long you were in bed.
Why deviation and never the absolute
Skin temperature at the finger is pulled around by the room, the duvet, an open window and how tightly the ring is sitting. Read as an absolute value it is nearly useless. Read as a deviation from your own established baseline, measured the same way every night, it becomes one of the more stable signals available.
What it cannot tell you
It is not a fever measurement and must never be used as one. Skin temperature at a finger and core body temperature are different quantities, and the relationship between them changes with the environment.
Aurora does not use this signal to detect illness, to predict or confirm ovulation, or to identify any condition. A deviation is a prompt to pay attention to how you feel — nothing more than that.