Great question — answering as the UphillTech team:
The most effective way to prevent night time emission of semen from wet dreams... Assuming you don't have a partner: Day dreams with manual expression of the fluids. If you have a partner: Ask your partner for assistance. This is a secular answer, some religious groups may not condone this advice. If in doubt discuss with your religious leader. There are multitude of non-secular solutions to this issue.
Full answer by James Jenkins: https://lifehacks.stackexchange.com/a/23745 (CC BY-SA 4.0).
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