Another study compared the response of surgically and medically castrated rabbits to vardenafil with that of control rabbits.  Castrated rabbits did not respond to vardenafil, whereas noncastrated rabbits did respond appropriately. This result suggests that a minimum amount of testosterone is necessary for PDE5 inhibitors to produce an erection.
It is normal for a man to have five to six erections during sleep, especially during rapid eye movement (REM). Their absence may indicate a problem with nerve function or blood supply in the penis. There are two methods for measuring changes in penile rigidity and circumference during nocturnal erection: snap gauge and strain gauge. A significant proportion of men who have no sexual dysfunction nonetheless do not have regular nocturnal erections.
Along the same vein as stress, relationship troubles may also be at the root of some men’s erectile problems. Not trusting your partner, fears of birth control failure, or just genuinely not being emotionally attached to a partner could make it hard for a man to have an erection, The Huffington Post reported. Talking about issues within a relationship can help to resolve this problem.