The liver dumps sugar in the blood early in the morning. The effect varies between people, but it's possible for instance that if your sugar is 6-7 in the morning, you were below 4 while you were asleep, whereas if you're 7-8 you were fine all night. To investigate you could set an alarm at 4am some nights, 1am other nights, or whatever, and test your sugar. If it rises a lot from the middle of the night to when you get up, this could well explain what's going on.