 Lethargy can be a normal response to inadequate sleep, overexertion, overworking, stress, lack of exercise, improper nutrition, boredom, or a symptom of an illness or a disorder.It may also be a side-effect of medication or caused by an interaction between medications or medication(s) and alcohol.When part of a normal response, lethargy often resolves with rest, adequate sleep, decreased stress, physical exercise, and good nutrition.