I started wearing a CGM and the default low alarm was 70… I woke at 4 am with a blood sugar of 68 and reset the low alarm to 65 and it gets to that range every night before I wake up.
95% of the time I am between 80 and (90 minutes after a meal) maybe 120.
I assume there is no actual danger until it is lower but that seems pretty low to me.
It depends on whether ketones are available or not to feed the brain. The lowest report I’ve seen on these forums was in the forties, if memory serves.
George Cahill, in one of his experiments that led to the publication of Starvation in Man, used a euglycaemic hyperinsulinaemic clamp to drive his fasting subjects’ serum glucose down to levels that normally mean coma or death, but they were fine, since they’d been fasting for several days at that point, and their ketones were elevated. As Stephen Phinney put it, in a couple of his lectures, “The only people sweating and feeling ill were the ones in the white coats.”
Of course, this type of experiment would never be allowed today by any IRB in its right mind, but it is illuminating.
That makes sense. My ketone levels using Keto-Mojo staying about 1.6 and I have no symptoms at least the ones I googled so… good to go.
My glucose gets down into the mid to upper 50’s now, but my ketones are high enough to make up the difference. That’s not during the night for me (as far as I know, no CGM), but when I hold off eating until around 3 or 4 in the afternoon.
Paul is right, as long as you have ketones and no symptoms, you are fine. It does shock one until you get used to the idea! 
Despite the experiment I mentioned, Cahill was convinced the brain needed 130 g of glucose a day. Benjamin Bikman has challenged that assertion, on the grounds that Cahill’s estimate, even though generally accepted, is not based on any known data. He has issued a challenge for anyone to present data showing how much glucose the brain actually needs. So far as I know, no one has yet met Bikman’s challenge.
It has been demonstrated that the brain actually prefers ketones over glucose when it can get them. Georgia Ede says, however, that certain parts of astrocytes are too small to accommodate mitochondria and therefore require glucose. So it is certainly possible that the brain might well need a certain minimum amount of glucose, no matter how abundant the ketones are. But Bikman’s point is that no one has yet produced any data to show that.
What we do know for certain is that the liver is perfectly capable of making whatever amount of glucose we do need (gluconeogenesis), so that dietary carbohydrate is not essential.
This is really a key point. Dietary protein is essential. Dietary fat is essential. End of list.
Dietary carbohydrate is necessary for neither health nor survival. On the contrary.
The most extreme example of non-harmful, non-debilitating low blood sugar I know of was in the famous case of Angus Barbieri, he of the 382 day fast. He registered a 17 mg/dl or ~0.9 mmol/l and he’d been walking around the city that day, going about his business, and felt fine.
In general, I’d really wonder about the 40s and certainly the 30s (around 2 mmol/l or less). But there are many people who feel fine in the 40s, 50s, 60s… And even among well fat-adapted people, some feel faint, etc., in the 70s - seems like there is a fairly wide range of how the given individual is going to feel.