Your metabolism should be measured in a basal (minimal functioning) state, or a state as close to basal as possible.
In order to ensure the most accurate test results and to create test conditions that mimic a true resting state, please follow these guidelines before the test:
Avoid eating at least 3 hours prior to the test.
Avoid exercising at least 3 prior to the test.
Avoid the use of stimulants, such as caffeine, for at least 3 hours prior to testing
A great time to take the test is in the morning, as close to waking hours as possible, before strenuous physical/mental activity.