One of the most important steps in determining when you should retire is figuring out what you want to do after you retire. Sounds like backward logic right? Most people think of retirement as the time in their life where they don’t have to do anything! But consider this - a 2012 study conducted by Elizabeth Mokyr Horner, Ph.D., of U.C. Berkeley revealed that many retirees experience a "sugar rush" of happiness shortly after retirement. However, that period soon fades and is followed by a significant decrease in quality of life.

The bottom line is that you could become bored if you don't have plans for what you'll do during your retirement years. It could be that playing golf or traveling will be enough to keep you happy and active. Even spending more time with family or a few days a week watching the grandkids may be fulfilling enough. However, many retirees go back to work in some capacity -- not because they have to, but because they want to.

Merrill Lynch surveyed working retirees in 2014. Four-fifths of them chose to work even though they didn't have to do so. The top reason they cited for continuing to work was "to stay mentally active." The second-highest response was "to stay physically active."

Volunteering with non-profit groups is another great option for many retirees. The important thing is to remain physically and mentally active, which has been proven time and again to be essential to retirees' happiness.