Why do employees work overtime?

I had written a post earlier about this but due to the 'number of words' limitation I had to cut short my original post. Here is the actual one I had penned down:

When assigned with a work we may be able to complete it within the 9-5 or we may have to work overtime. But working overtime everyday or most of the days? Do working more hours show that you are more dedicated to the job? Does longer work mean more productivity?

Here is a bubble-burst for the employers who believe that working hours is directly proportional to productivity (also for some of the employees who believe the same and have not yet realized that it doesn't work that way):

Working long hours won't make you more efficient. Productivity might be high for the first few days; then after that, it will gradually fall. Working past your productivity hours on the long run will have adverse effects on the mental health and work-life balance:

1. Stress/anxiety
2. Fatigue
3. Depression
4. Burnout
5. Heart problems

...to name a few. And to top it off, downfall of your personal and social relationship.

Employees tend to procrastinate the work assigned when they are made to work overtime. It is not rocket science but basic human behaviour fact we all know. They tend to work smart when they are given a shorter period of time to do an assignment. This increases productivity and that leads to more resting hours, which in turn leads to a happy employee with a happy tomorrow.Now we all know that a happy employee is a happy employer with a higher outcome and profit.

A note to employers: Invest in employing additional human assets instead of depreciating the ones you already have!