If you mean whilst they're sleeping and during the morning, read:
"A man's penis becomes erect in response to sexual arousal and excitement, but it can also become erect as a result of deep relaxation, such as what's experienced in deep sleep with or without dreams. An erection is caused by a rush of blood to the penis. A man's penis, especially a young man's penis, can also become erect without any kind of arousing stimulation this is called a "spontaneous erection."" [ thelaura's advice column | Ask thelaura A Question ]
Attention: NOTHING on this site may be reproduced in any fashion whatsoever without explicit consent (in writing) of the owner of said material, unless otherwise stated on the page where the content originated. Search engines are free to index and cache our content. Users who post their account names or personal information in their questions have no expectation of privacy beyond that point for anything they disclose. Questions are otherwise considered anonymous to the general public.