Why would exercising late in the day cause a spike in blood sugar the next morning?

I hope this doesn’t cross the line when talking about health-related issues. I’m not really looking for medical advice, as the problem I’m about to describe has a trivial and obvious solution which I’ve already implemented; I’m just interested in the mechanics.

I have type-2 diabetes. When I was first diagosed I managed it with glyburide, Avandia, diet, and exercise; now I’m down to diet and exercise, and haven’t taken any any meds in a few months. I do an hour on the bike or the treadmill each day, and don’t allow myself to eat anything after 7 p.m. unless my blood sugar is elevated right before bed. My A1C numbers are good – under 5.9 the last two times I was checked.

When I went entirely off pills a few months ago, I was doing my hour of exercise all at once: in the morning before breakfast and after checking my blood sugar. Under this regime my morning reading would be between 90 & 110 – pretty much exactly what it should be – and my readings during the day would be no more than 130 or so unless I pigged out. These were similar to the numbers I got when I was exercising half an hour in the mornings and taking a glyburide pill.

In December I found myself getting busier in the mornings, so I changed my exercise routine slightly: half an hour in the mornings, then half an hour in the evenings after dinner–generally around 8-9 o’clock. Other readings remained fine , but I noticed my morning readings creeping upwards to as high as 140. I changed back to the previous routine on New Year’s, and my fasting reading went back to what it had been.

I did NOT indulge in sweets & such during the holidays, so that can’t be it. What could cause the change I’ve described? I mean, I realize that my liver must be releasing glucose during the night, but I’m not sure why. Thoughts, anybody?

My gut guess would be The Somogyi effect since you weren’t eating after 7pm and then participating in activities that can both lower blood glucose and improve insulin sensitivity. Your blood glucose was possibly dropping too low as you slept, which led to signals being sent to your liver to ramp up glucose production/glycogen breakdown. Net result the next morning: abnormally high fasting glucose levels.

Thanks! I’ve bookmarked that page and will research the topic later.