When you become tired, your brain politely and quietly tells you to take a break.  If you ignore the message, the mind becomes more aggressive and starts displaying negative thoughts to get your attention.  If you continue to ignore the message, the mind takes drastic action and hits you right between the eyes with a major dose of negative thinking.  It will continue pummeling you with negative thoughts until it gets your attention, and you stop and rest.  The mind has no other way of informing you that you are in a state of temporary fatigue.  The mind is tired of taking the abuse, and it will do whatever is necessary to get your attention.  Negative thoughts are there as a warning.  If you heed the warning, the negative thoughts go away.  When the mind is rested, it no longer sends uncomfortable negative messages.

Many people don't understand these warnings.  They struggle with their negative thoughts, and they don't understand why they feel bad.  They fear that they are suddenly becoming a negative person.  As they continue to push forward, their negative thoughts become more exaggerated.  Instead of being transformed by the renewing of their mind, they are negatively transformed by the fatiguing of  their mind.  They ignore the distress signals sent up by a tired mind.  The solution to this problem is simple when the person understands that the negativity is only a sign of fatigue.  They aren't suddenly turning into a negative person with a negative operating system.  All they need is rest and sleep.

Excerpt from :

Zero Tolerance To Negative Thinking : Good-by Depression - Hello Positive Mind

David J Abbott M.D.