Temperature, time of day, medications, activity level, and what you've had to eat or drink are all factors that can affect your finger size. Finger size is most stable after lunch and before dinner.
Cold temperatures cause fingers to shrink and warm temperatures cause them to swell.
If the ring sizer is snug during the day, that size might be uncomfortable at night when fingers tend to be more swollen.
It is crucial that a ring fits tightly over the knuckle—so it doesn't fall off the finger—but is not so tight as to restrict blood flow. When the knuckle is bigger than the base of the finger, measure both places and pick a size in between the two.
Wider bands (from 6mm) tend to fit more snugly, so consider ordering a quarter to a half size larger in this instance. Chain rings run small, so order a full size larger than your measured ring size.