In the United States, it is not usual to telephone someone early in the morning. If you telephone early in the day, while he is shaving or having breakfast, the time of the call shows that the matter is very important and requires immediate attention. So it is with the telephone calls made after 11:00 pm. If someone receives a call during sleeping hours, he may think that it's a matter of life and death. The time chosen for the call communicates its importance. In social life, time plays a very important part. In the USA guests tend to feel they are not highly regarded if the invitation to a dinner party reaches them only three or four days before the party date. But it is not true in all countries. In other areas of the world, it may be considered foolish to make an appointment too far in advance because plans which are made for a date more than a week away tend to be forgotten. The meaning of time is not the same in different cultures that treat time differently; being on time is valued highly in American life, for example. If people are not on time, they may be regarded as impolite or not fully responsible. In the USA no one would think of keeping a business friend for an hour; it would be too impolite. A person who is 5 minutes late is expected to make a shout apology. It he is less than 5 minutes late, he will say a few words of explanation, though perhaps he will not complete the sentence. A call at midnight would mean  _  .
it is a matter of life and death the matter is too difficult to handle the matter requires immediate attention the matter is less important