There is no standard answer, many factors determine that

If you don't use insulin or take sulfonylurea's and need to test to prevent an accidental overdose then how often or when you test is determined by what information you are looking for

If you are simply interested in your fasting numbers then one every morning or every couple/few mornings may be sufficient

If you are trying to determine if the meal you just ate is blood sugar friendly for you then you would test two hours after

If your Doctor has not given you a specific schedule then test when you feel the need

And yes, high blood sugar can cause sleepiness particularly in the early afternoon. It is a metabolism thing that happens when sugar levels are high and insulin resistance is high so you can't convert that sugar into energy

Apple 🍏 cider vinegar instantly lowers your blood sugar also

I now Have a dexcom meter that reads the blood sugars automatically When I have a high blood sugar reading, I take cinnamon capsules which helps lower the sugars. Talk with your Doctor about taking cinnamon I used to take my blood sugar readings 3 to 4 times a day.

Thanks for the valuable comments and advice....much appreciated!!
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I don't test all the time, however, if I'm feeling off, I'll test my diabetes first to see if it's high or low, depending on how I'm feeling. It might not be your diabetes, but pain, stress, or something you've eaten. Always good to test if you are feeling off, then you can manage the problem.