A 2012 article in the Psychology Today magazine defined feeling burnt out as:

"...a state of chronic stress and frustration that leads to:
• physical and emotional exhaustion;
• feelings of cynicism and detachment; and
• a sense of ineffectiveness and lack of accomplishment." See: http://m.psychologytoday.com/blog/high-octane-women/201205/where-do-you-fall-the-burnout-continuum

Whilst you may not be experiencing all these symptoms, if you are regularly feeling mentally drained at the end of the day, it's time to start having a look at how you are processing information throughout your day. For instance, if you engage in a lot of negative thinking throughout the day (such as complaining about people that upset you at work), you are straining your mind and filling it with very draining thoughts. These thoughts then lead to negative feelings.