It actually correlates to your stamina value.  However much stamina you have at any given time indicates how well rested you are.<br/><br/><div class="quoteheader"><div class="topslice_quote">Quote</div></div><blockquote class="bbc_standard_quote">Top Shape (81%-100% Stamina Values)      -1<br/>Well Rested (61%-80% Stamina Values)       0<br/>Tired (41%- 60% Stamina Value)               +1<br/>Fatigued (20%-40% Stamina Values)          +2<br/>Near Death (0%-20% Stamina Value)         +3<br/>Notes: Should your HP fall to about 15% of its base value, you'll be at a Near Death Scenario</blockquote><div class="quotefooter"><div class="botslice_quote"></div></div><br/>Makes sense too.  People with lots of stamina and constitution will be able to do more without tiring than other people.