if energy cannot be destroyed, why do I get tired?

Question: if energy cannot be destroyed, why do I get tired?
- Keywords:
  chemistry,
  response,
  thermodynamics
Asked by meep to laurenceharwood on 9 Mar 2012.
- Laurence Harwood answered on 9 Mar 2012:
  
  Well I’ve just got off a trans-Atlantic flight and I’m shattered!!!!!! However, what we refer to as “energy” in our bodies refers to stores of fuel – sugars and fats – that can be used by the body to carry out all the things we do – consciously and unconsciously. When those reserves run low we feel tired. If they run low too quickly, a substance called lactic acid builds up in our muscles and makes them ache.

Question: if energy cannot be destroyed, why do I get tired?