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Other than in situation you are in deep sleep (bhavanga) you create Kamma. Since when you are dreaming you are still creating karma as you cannot dream in deep sleep. These are not very strong or deep kamma and in most cases may not give any results and if they do they will be very mild (maybe like an itch which we might experience then the result comes). They do not have potent to create a new rebirth. For this to happen it should be backed by action or word giving desired result. More particularly:
Kamma creation to a dream is based on reaction you can react:
These reaction can lead to mental kamma.
In many cases as these kamma has very little potent, it can be ignored held there is no kamma created due to dreaming like in this answer.
Additional references that might be of interest:
Upvote:2
According to Buddhism, to do a sin, you have to consciously do something using your body, speech or using sitha.
In a dream you are not doing anything consciously. Therefore simply you can not sin in dreams.