If metformin is taken daily, it is to be taken with the evening meal. The main reason metformin has to be taken with food is the reduce the risk of GI upset. It is recommended to take it in the evening because the medicine acts on the glucose generated from your liver. The liver generates glucose on its own to provide energy to the body in between meals. Generally, the longest period of time a person goes without a meal is at night so this is when the liver will be actively producing its own glucose. Therefore metformin will work best during this time.
Metformin does not always last a full 24 hours for every patient. When this happens it may become necessary to split the dose between morning and evening. I have read of many diabetics on the diabetes sites who are doing this, and it has helped them very much.