The answer is....Physically they can do it the next morning...
But why they don't?
How long do we grief?..Think this way..if the next day after you lost a loved one,and you didn't grieve..woul'dnt that cause a great deal of pain in your life?
First People would immediately believe you didn't care..for the person...
The fact that we choose the pain of grieving..rather than changing our emotions,until we're satisfied..that our standard of rules and cultural standards..about what's appropriate have been met..
In Fact there are ..cultures where people celebrate and laugh when someone dies !!!!!
I DONT SAY GRIEVING IS BAD OR WRONG...I JUST WANT TO MAKE IT CLEAR I'TS ..I'TS OUR BELIEFS..THAT THE EMOTIONAL PAIN..TAKES LONG TO RECOVER...EVEN A LIFE TIME