Fatigue is a feeling of extreme tiredness. We can divide fatigue into three types by its causes. First, there is physical fatigue. The cause of this fatigue is the short of energy to move muscles and accumulation of lactic acid, nyusan in Japanese. Muscles do not work properly without energy like a car going out of petrol and with much lactic acid accumulated because they are easily affected by acid. For example, you are tired after running long distance because your body uses almost all energy for running.

Second, there is mental fatigue. Annoyance and stress are big cause of this fatigue. For instance, when you are with your boss or teacher, you should be considerate of them and may get nervous. This stress makes your body and mind tired. If worse comes to worse, mental fatigue causes depression.

Finally, there is neurological fatigue. This fatigue is brain fatigue. When you use a smartphone or a computer for a long time, it can strain your eyes. Because eyes are part of brain, it is also burden on brain. Brain’s fatigue causes lack of concentration and declining enthusiasm and the ability to think.

In conclusion, physical, mental and neurological fatigue is connected closely and cause serious illness if you leave them without a break.