Research in the United States shows that the failure rate is 15% to 28% per annum. And, both men and women are frustrated by having to interrupt at the last moment. Lack of sleep builds stress and lowers testosterone in your body. This is the chemical produced in the body which allows the blood vessels to relax and allow sufficient blood to flood into the penis and create an erection. If you don't get enough of it, you won't get an erection, so top it up: Sperm may be found in the "dewdrop" of the liquid that one produces when excited, but there may be sperm before he ejaculates.