Welcome to NeuroTalk:

Has your husband had testing for B12 levels? This needs to be done, especially now because metformin blocks B12 absorption.

There are some studies showing that sleep impairment, can affect blood sugar levels. When people cannot sleep, it may be due to poor melatonin synthesis. Melatonin synthesis is cofactored by methylcobalamin--the active form of B12.

If he is using acid blocking drugs alot for GERD, this also depletes B12.

So being low in B12 can have a serious impact on sleep quality.
Low B12 can also cause nerve damage, so getting tested and
being at least at 400, is the first thing you should examine.
Don't accept "normal" from a doctor, as lab ranges in US still report levels lower than 400 as "normal" when in effect they are seriously low.

This is my B12 information thread:
http://neurotalk.psychcentral.com/thread85103.html

Also I would like to add that people who crave sweets may have episodes of LOW blood sugar. Since your husband is on metformin I'd make sure he is using a glucometer (preferably One Touch) and testing several times a day, to see if he can find times when he is very low.
Waking up at night with low blood sugar, and then hence a craving to eat is very common. It can be accompanied by vivid dreaming or nightmares which wake the person up. Also night sweats would signal a low period. Testing then may reveal lows which are important and should be addressed. Sometimes a small snack before bed helps. Either Glucerna (which is designed for diabetics) or a protein bar may help with the wakening. I would do alot of testing for a week or two to find trends with his blood sugars. It can help tremendously in the long run.

One Touch works with a chemical system that does not give false highs when other types of sugars besides glucose are ingested, so I think it is the most useful of the meters. You can get a free One Touch mini online. Most insurances pay for the strips (which are very expensive).