I have always used the 728.87, but after reading this post I looked at 780.79 on my coding software and this is the definition it gives for the code;
DEF: Asthenia: Any weakness, lack of strength or loss of energy, especially neuromuscular.~DEF: Lethargy: Listlessness, drowsiness, stupor and apathy.~DEF: Malaise: Vague feeling of debility or lack of good health.~DEF: Postviral (asthenic) syndrome: Listlessness, drowsiness, stupor and apathy; follows acute viral infection.~DEF: Tiredness: General exhaustion or fatigue.
So I think I will be using this code from now on. Thanks.