I have experienced this, that’s why I called it a Big Resistance in Concentration learning process. This Resistance is Feeling Sleepy. You will be feeling Sleepy while trying to concentrate or while doing exercise of concentration. I have tackled this problem. I am sharing my experience here. If your option is different, please share with me, I shall remain grateful to you. In the exercise, you will fall asleep or you will take a nap unknowingly even if you have got full sleep at night. In my view- it happens to everybody. Once, while I was sitting for concentration I nearly fall back because of sleep. If you are overwhelmed by sleep, please try doing Shavasana. At such situation please do not watch flame of candle, instead watch your breath. Sleep on a mat or a bed and watch your breath as mentioned in previous story. Then you will slowly go into sleep, no worries. Continue doing concentration for at least five minutes after you wake up taking nap. It may take time but you will overcome such annoying Resistance, too. Always keep hope.