Simple answer is Hormones.

There is a handful of hormones that drive our blood sugar numbers.

The two main ones are Adrenaline (epinephrine) and Cortisol (a growth hormone) but there are a number of others (thyroid stimulating hormone and a couple others released by both the thyroid and/or pituitary gland) that will cause fluctuations as well.

In females estrogen and progesterone can cause massive fluctuations.

So it's almost never the "what" causes it but rather "why were they released" can become the question.

Simply the process of waking up causes the release of Adrenaline and Cortisol - and the harder it is to wake up (or a number of false wake-up's when we sleep poorly) the more of those that is released triggering a release of extra stored sugar in our system.

Other factors such as stress or pain or fear also cause hormone responses that trigger sugar release.

All of these variations can be seen as "normal function" however, if a trend develops, typically with your numbers trending either up or down and meds no longer can control that (as part of our typical up/down cycles), that is when an Endocrinologist (hormone doctor) can help figure out if there is a secondary issue causing an abnormal hormone response which is messing with our blood sugar.

Two of the most common hormone disorders that can significantly effect our blood sugar is hyper or hypothyroidism (thyroid disorder) or Cushing's Syndrome or Disease (a disorder in the adrenal glands). Rounding out the Top 3 would be a Pituitary gland issue.

Your test is a moment in time. Your blood changes moment to moment as your body uses or make glucose!

Also meters can be as much as 20% off either way. So if your actual level is 100 your meter could show anywhere from 80 to 120. The higher you go less accurate it is. (This is explained in the fine print that comes with your meter) this inaccuracy is why most drs do not treat sooner as you could go too low if you medicate solely on one reading.

Hi @A DiabetesTeam Member and all you warriors. For me, I try to control in a very tight range. When I suspect I might be higher than my normal I adjust with little tricks to make me release more shots of insulin.

I find that between 3-5 diabetes friendly whole fruit carbs trick my pancreas to release more insulin than is needed to handle the fruit carbs. I also find that 4 whole grain bread carbs has a similar effect with a net drop of 10-15 points USA after 3 hours. The 2nd time the net drop is only half that.

For me it is relatively easy to do this at night since I have to visit the loo every 1.5-2 hours overnight. Post prostate cancer surgery 5 years ago, and 7 treatment regimens since. The cancer continues to grow back in different organ systems and locations since the start.

This is a bit of a minor inconvenience and irritation, but a small price to pay as I continue our traveling lifestyle and adventures since retiring 90 days ago.

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A Diabete...

Remember it takes a few hrs for our food to leave our stomach and sugar gets absorbed in intestines. According to gastric emptying tests, at 1 hr after a meal you should have have less then 90 % of food left in the stomach, after 2 hrs you should have less then 60 %, at 4 hrs less then 10 %, hence why they make us check our sugar level at 2 hrs where most of the food should have left the stomach and nutrients such as sugar mostly gets absorbed in the intestines. So diabetics, we have more sugar circulating in our blood instead of going into the cells, the only thing which can pull the sugar out of our blood is insulin we produce, but as diabetics our insulin production and absorption is malfunctioning, so the sugar stays in the blood stream not entering the cells, hence the fatigue, cramps, circulation issues and all the side effects of being diabetic. The liver tries to do its job to make sure the cells in our brains do not lack sugar so it can keeo us going, but it has no clue our pancreas is defective,and this is where us diabetics get into trouble. If people skip breakfast it just makes your liver work harder by releasing more sugar trying desperately to send enough sugar so our bodies do not shut down. So beacuse it takes a good 2 hrs before the food we eat can supply us with some sugar, fasting does not help at all, the longer you are without eating, the higher your fbs will go.

I have 3 medications which require me to wait 30 min to 1 hr after taking them before I eat, and my fbs when waking up till I can actually eat fluctuates so much, if it were not for the fact my meds have to be taken fasting my numbers when waking up are decent, but just that 30 min wait the levels go up, the real wait time for one of my meds is 1 hr (but as pharmacist stated because I am diabetic at times eating 30 min after might be best for my levels to come down. Hence why I check my fbs just before I eat, but when I had a libre on the numbers can fluctuae by a whole 1.0 (USA 18 points), The true fbs is right before you eat, not when you woke up as in my case the numbers change a lot. Also this is why doctors want a 12 hrs fasting level along with the A1C to see how high your numbers go after 12 hrs of not eating (as I stated nothing you eat the night before will affect your morning numbers, it is the battle between the liver and pancreas they are checking out, diabetics our pancreas cannot adjust the sugars as well. I remember when working on surgical floors as a nurse, we had to put patients on insulin drips while they were fasting for surgery, or else their numbers would go up by a lot, at the time (20 yrs ago) I did not understand why if a patient is not eating they required insulin drips, but now I understand diabetes a lot more from what I have learned in here mostly, the patients on the insulin drips required testing their sugar levels often and doctors would adjust the dosage according to levels. Now it makes sense to me.

This is probably best answered by a health professional but I believe it has to do with what or when you last ate. A fbs depends on a long enough period without food or drink of any kind but not too long of a period either.
Hope that helps some. 😅