Short answer: Hypoglycemia unawareness means not noticing the warning signs of low blood sugar until it has dropped very low. The NIDDK says repeated lows can lead to it. Insulin labels add long-standing diabetes, nerve damage from diabetes and beta-blocker medicines as reasons warning signs may be weaker. In small studies, strictly avoiding lows for weeks to months brought warning signs back.
A low usually brings early warning signs. The NIDDK lists feeling shaky or jittery, hungry, tired, dizzy, confused or irritable, a fast or uneven heartbeat, and a headache. These give a person time to treat the low before the brain is affected.
The NIDDK describes hypoglycemia unawareness as "a condition in which you don't notice any symptoms of low blood glucose until your blood glucose level has dropped very low." By then, the person may not be able to treat it alone. The NIDDK also says lows during sleep can make you less likely to notice and respond to symptoms during the day.
The Lantus, Humalog and NovoLog labels (section 5.3) say awareness of a low "may be less pronounced" in people:
The Humulin N label (section 7) puts it more strongly for beta-blockers, clonidine, guanethidine and reserpine: signs and symptoms of a low "may be reduced or absent." The NIDDK adds alcohol, which can keep you from feeling the first symptoms. More on that is in insulin and alcohol.
Two small studies tested whether unawareness can be reversed. In Fanelli 1993, eight people with type 1 diabetes for about 7 years and hypoglycemia unawareness worked to prevent lows for 3 months. Lows fell from about 0.49 to 0.045 per person per day, and HbA1c rose from 5.8% to 6.9%. After 2 weeks, symptom responses during a test low were nearly normal; at 3 months, the blood sugar level at which symptoms started had returned to normal.
In Cranston 1994, twelve people with long-standing type 1 diabetes and lost awareness avoided lows for about 4 months. Hormone and symptom responses to a test low rose, and they started at a higher blood sugar than the point where thinking became impaired. HbA1c did not worsen significantly. Both studies were small, but they point the same way.
For people with reduced awareness, the insulin labels recommend checking blood sugar more often, and they advise people with frequent lows or weak warning signs to use caution when driving or operating machinery. The NIDDK says a continuous glucose monitor, which can sound an alarm when glucose drops below a set range, may be a good option for people with unawareness or frequent lows.
How to change targets or doses is a decision for the care team. A log of lows, including the reading, the time and whether you felt anything, is what they will want to see. Dose Track keeps those notes next to your doses so patterns are easier to spot. The hypoglycemia entry covers the basics.
Read more on Dose Track: Hypoglycemia (low blood sugar) · HbA1c · Lantus side effects · Blood sugar