Your blood sugar of 4.0 (72) is not considered low, but your 2.8 (50) is. Hypoglycemia is usually caused by too much insulin, too little carbs or a combination of the two. You don't say anything about your insulin regimen. Are you on a basal/bolus regimen? How do you determine your bolus (pre-mea) dose? The best way is by finding out what your insulin:carb ratio is so that you gear your dose to what you plan to eat. If you are going low BEFORE your meal, your basal (long-acting) insulin dose may be too high. I highly recommend getting the book Using Insulin by John Walsh which will answer many of your questions. If you pay attention you may start to recognize when you are low by such things as shakiness, irritability, fatigue, etc. Some lows are unavoidable especially if you want tight control, but proper insulin doses and carb counting can avoid most.
I am on insulin. I use 20ml of Lantus and 11ml of Novarapid with meals. These are the doses I was told to take by my Doctor. I am waiting for my first appointment with a dietician at the hospital and I need to see her because I have been told nothing about how to count carbs. My Doctor said that the dietician will explain all of that to me when I see her. I have experienced lows before and I had plenty of signs, shaking, sweating, feeling wek etc., but with the 2,8 I felt nothing at all, perfectly normal. I spoke to me Diabetic Nurse this afternoon and she has told me to cut my Novarapid to 9ml before that meal and to take a small snack in the afternoon. I am going to see her on December10th. Here in UK. Healthcare is free which is great, but you do sometimes have to wait a while to get appointments. Thanks for the advice.