2. Seeing that nail while sleeping will attract people's attention and affect the quality of sleep. Psychologically speaking, it will give people a "danger warning", worrying about hitting their heads and being uneasy.
3, but it doesn't matter if the nail is hidden behind and can't be seen. For example, many people will hang wedding photos on their beds.
4. However, it is best to hang photos with lighter texture. Don't hang it on your head when it is particularly heavy. Feng shui is called "beheading", which is unlucky, because if you fall, you will die.