Watching the Deceased; The deceased should be watched constantly at all times until his burial unless the deceased is put in a box in which case he only needs to be watched from time to time. The family should arrange for a shomer (watcher) to be at his side at all times up until the time he is put in a box. It is preferred that the shomer be a member of the family or a personal friend. The funeral director or Rabbi should be able to make arrangements for a Shomer, but you should ascertain whether or not he is reliable, since the Shomer must watch the body and recite Tehillim (Psalms) day and night until the body is buried and the Shomer cannot fall asleep.
a. One should refrain from idle conversation or frivolity while watching the deceased. It is best to recite Psalms while watching the deceased. Work, homework, etc. should not be performed while watching the deceased.
b. While watching the deceased one may only recite Psalms. One is not permitted to learn Torah, wear Tefillan, wear Tzitzis in the open, or engage in any other mitvos in the same room where the deceased is lying, or within 7.5 feet of the deceased in an open area where the deceased was placed.
One of the reasons is that it will remind the soul that he/she can no longer perform mitzvos on this earth since they have recently passed away. However, by taking on mitzvas in general while not in the presence of their body or after they are buried, is a tremendous source of pleasure and an elevation for their soul.
Also, while watching the deceased an onen (mother, father, spouse, son, daughter) should not eat, drink, give a greeting, or smoke in the same room where the deceased is lying, or within 7.5 feet of the deceased in an open area where the deceased was placed.
 Shach 341:18
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