I'm researching the mechanisms of interactions of sildenafil or viagra in my decision whether to take it to solve my insomnia (I sleep only 4 hours a night). I found out that Nitric Oxide (NO) is released with sexual stimulation from nerve endings in the spongy erectile tissue of the corpus cavernosum of the male organ. The enzyme guanylate cyclase then converts guanosine triphosphate (GTP) into cyclic guanosine monophosphate (cGMP). cGMP causes the smooth muscle to relax, which causes an inflow of blood which then leads to an erection. cGMP is then hydrolysed back to the inactive GMP by phosphodiesteras type 5 (PDE5). Sildenafil or viagra disables PDE5 which prevents the breakdown of cGMP. Now there is a part I can't figure out. If sildenafil or viagra inhibits PDE5 for 4 hours which means the cGMP didn't break down for 4 hours. How come after an initial 30 minute sex, the male organ became soft again when the cGMP is still active for another 3.5 hours even though no Nitric Oxide is being sent anymore? Could cGMP self break down to GTP in a certain time without the presence of PDE5? If not, what cause the organ to get soft when cGMP is still active with the PDE5 still inhibited for another 3.5 hours or so?