In the article linked by mrsD, the dosage of melatonin given to the women was 5 mg. This is significantly higher than the doses of 500 mcg and 1 mg that I had used before, which is possibly why I didn’t notice this type of effect on my blood sugar. It is quite clear that melatonin did have a negative effect on the test subjects:

“The effect of melatonin on the insulin response to the OGTT depended on the time of day (P < 0.05). In the morning, melatonin decreased glucose tolerance primarily by decreasing insulin release, while in the evening, by decreasing insulin sensitivity.”