In some countries people never take photos of those on sleep and here is why. When the photography just started to develop, folks would take photos of their dead relatives just before the burial. Presently, such things are not commonly done, but at those times, that would be the only visual memory left for them.
Surely enough some sleeping people greatly resemble the dead ones. That is why it is deemed to be the bad omen to take such photos. They can come true after all. However, that is not the main reason to avoid photographing sleeping folks.
There is a theory that teaches – the soul leaves the body in sleep. So, a person becomes highly vulnerable to demonic powers. Photographing them may unbalance things and cause problems in the person’s life or even prevent the soul from coming back into the body. Or they may become even more vulnerable to evil spirits.
Finally, there is the ethical reason. When you are asleep, you do not control your body. That is why such photos turn out so funny. Would you want to be caught off guard and made fun of when you sleep? Surely not. So, you got now three good reasons to stay away from taking pictures of those asleep. Pray for them and wish them well instead.