When you are sick or heartbroken and too tired to get out of bed, the listlessness you feel is called languor. It's sluggishness and slowness, but usually with cause.
The more commonly used word languish is closely related to languor. If you are languishing or becoming weaker, you are showing languor. Remember that languor implies a heaviness or slowness where there should be lightness or speed. There is no time for languor if you have an exam tomorrow, and you are just beginning to study now. And, a 100 degree day with 70% humidity can inspire languor in just about anyone.