Although the science on this is hardly firm, the theories are most interesting. First of all let's call Morning wood by its scientific name-- nocturnal penile tumescence . It's a perfectly natural occurrence experienced by healthy men 3-5 times a night regardless of what kind of dream they have. It is theorized that the body triggers NPT to stretch out the cavernosum (the two sponge-like regions that fill with blood to cause erection), something that helps prevent bed-wetting and erectile dysfunction.