Usually the brain will start to feel discomfort starting with 70 mg/dl. Usually the brain secretes adrenalin in response to the irritation of brain activities caused by the low blood glucose. Over time our brain is getting used to these situations. So the first level response like shacking hands and sweating will disappear over time. Same is true for people working in very stressful job environments. The adrenalin deposits have to be refilled after they have been secreted. This might take up to one day. Thus the second low within 12-24 hours will be detected much later due to the low level of Adrenalin available. So overtime we are faced with declining reponses and depending on the reccurence of lows we can have no first level signs of being low at all.
Luckily there are still second degree sings of being low. These are the side effects of functional impairments of brain activity. This is much later than the first level reponse - about 50 mg/dl or lower. Visual distortions combined with feelings of fear and weakness are strong indicators. I try to focus on these to catch the low. Now the window to actually fight the low before your consciousness is getting impaired is starting to close. The meter will start to get unreliable 50 can be 40 can be 30. This is also because your body is switching to central blood circulation. The extremeties like the hands will get less blood flow. Thus the numbers measured there can be behind the actual development (5 minutes or more). It might help to shake your hands to increase the blood flow with very cold hands. But better focus on fighting the low than measuring it. Here it really helps to have a ritual at hand to get the number of glucose tabs needed. I usually cut off 3 tablets with my finger nail from the paper packed tabs and eat them. This way I do not get confused with the counting process while being low.
As others wrote the rate the BG drops is also important. It can be very irritating to feel the BG dropping. Especially in the colder periods of the year the BG might drop very fast while your tested finger might only report low 80 mg/dl. In these situations I have learned to trust my feelings more than the exact number on my meter.
Being able to function properly at low glucose numbers can be seen as robustness or resilience. But due to the reliability problems with current meters and the unreliable measurement process itself I would not trust on that. 30 mg/dl at your finger tips can mean that your brain is already at 20 mg/dl (0,020g). Your brain needs about 140g of glucose/day = 6g/hour = 0.0972g/minute = 97mg/minute on average! So your brain consumes 97mg every minute while your heart is pumping 20mg per deciliter through your brain. Obviously this will not work for long. Other consumers like the muscles and heart need glucose too. It is a small window as I wrote before and the glucose will deplete very quickly. This will end with a seizure if not treated aggressively. The likelyhood to survive this are not that bad it seems - but we better do not test that out. I did that once in over 25 years with T1 and it was enough for my taste.
