Nightfall in man is a phenomenon where he ejaculates (releases sperm) in his sleep, mostly late in the night or early morning. This is common among teenagers and adults, caused due to weakness of the nerves and muscles of the penis, fluctuating hormonal levels, and other factors.
Our erections are much more than just blood flowing to our penis, it's also a matter of our self-esteem and pride. For many, it takes a while to get an erection. For others, it's hard to maintain it through intercourse.