I think it’s more like mravg said. What is often overlooked with nocturnal erections is their frequency and duration in healthy men without ED - four to five of them a night, each lasting about 30 minutes. That is a heck of a lot of erection time and obviously there is a physiological benefit to that pattern. Jelking does surely improve arterial blood supply over time. Most of the guys here (who do not also jelk themselves into oblivion and tissue exhaustion) report improved erectile function.
But if you have ED and want a quicker result in kick-starting nocturnal erections, Viagra is probably more effective than jelking. Cialis can work as well in that way. Levitra (more similar to V in effect) probably does too but I never got past the first try on that one. Brutal heartburn for me.
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avocet8