It tends to be a more important issue for dogs than for cats. Dogs are often taken out for walks, playtime, or go for day trips with their owners. Cats can be encouraged to exercise, or you may play vigorously with your cat for a short time, but cats usually just do their own thing. For both cats and dogs, it is important to keep their activity level the same from day to day. The time of day when exercise is done, how vigorous the activity is, and how long the activity lasts all effect the bg levels.
Exercise generally causes the bg to decrease.
There are many reasons for this. Exercising muscles need energy, and they get that some of that energy from blood glucose. As the animal exercises, the blood glucose decreases. Exercise also increases the heart rate and increases the blood circulation. The increased blood circulation causes insulin to be absorbed more quickly, which causes the blood glucose to drop.