Natural process at work, the liver will release glucose into the system to keep everything running, especially if no food has been eaten, plus, mornings are when the liver will do this anyway. It allows us to get up and function and find food. Afternoons, you've already got food in the system, a walk will lower levels as the liver does not need to do what it does in the morning.... unless, you run or do something demanding on the systems and there is a sudden need for extra energy to be suddenly released. Fight or fight response.