Re: Why do people weigh less in the morning than in the evening?

Erin,

You are correct in your statement that matter must be conserved in any 
closed system.  When you weigh yourself in the evening and then again the 
next morning, however, you have not maintained a closed system all night.  
Weight losses include any urine or other body wastes, plus the things that 
are less obvious.  The normal daily urine loss at normal temperature is 
approximately 1400 ml/day or approximately 3.3 pounds.  When your body 
converts food materials to energy, CO2 and water vapor are released to the 
blood stream that ultimately may leave through the lungs.  Water lost 
through the lungs amounts to approximately 350 ml/day at normal 
temperatures or approximately 3/4 pound.  The skin loses another 350 
ml/day to insensible perspiration (that which you do not notice as liquid 
water but evaporates directly into the air).  The sweat you do notice as 
liquid is lost at a rate in proportion to the surrounding temperature and 
the level of activity.  Normal temeprature with little activity can 
produce approximately 100 ml/day this way while under prolonged heavy 
exercise this number may go up to 5000 ml/day or 11 pounds.

The daily totals for water loss during a normal day add up to between 2400 
and 6700 ml/day or from between about 6 to about 15 pounds per day.  If 
1/3 of this occurs at night, the body weight loss could be in the range of 
2 to 3 pounds.

Textbook of Medical Physiology
W.B. Saunders Co.
Philadelphia, PA
1976