If a company allows for a short nap (Say 15 minutes) for its employees, do you think this will: waste time,Increase or decrease effectiveness of work?

I don't think a15 minutes nap will be enough for some people. So definitely it's a waste of time.

I am in favor of middle break either for lunch or prayer but15 minutes are not enough this should be at least half an hour. This is important for the employee performance because continues work stress will make them sick and their mind will be full of tensions rather to go for eat, drink or prayer. Wasting of time will  not be happened if the manager distributes the time schedule by adding extra30 minutes for mid break. Money loss will  not be happened if the manger prepare two or three groups in which one group go for break the other continue and when the first group arrive the second should go for break. A happy employee means a happy work atmosphere where fresh energy of workers really boost the final fruitful results of business, work, tasks and assignments. A tired worker will definitely look for relaxation and without any break means low energy from the workers at work place which will really drop down the progress and also God never like those who put or choose more pressure, stress, hard targets for his workers. Better give care and get best results with satisfaction of everyone.

The answer to this question depends on many factors related to company size, kind of business, load of work...

Small companies don't even like to give their employees a one hour lunch break

Sales and customer service employees have to be ready nine hours per day to provide service to customers. Maybe during this fifteen minutes a large deal will be lost.

Premises in most companies do not have rooms and places for rest of employees.

Employees themselves need to prove themselves and to finish work on time to go home to their families and do not need a fifteen minutes nap.