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''The physiological mechanisms'' underlying the beneficial effects of head-up sleeping are a decrease in sodium and water excretion during the night resulting in a better maintained and plasma volume; an increase in body weight of  0.5- 1 kg is reported <cite>6</cite><cite>8</cite><cite>9</cite><cite>10</cite><cite>15</cite> (for review see <cite>16</cite>. The observation that head-up sleeping becomes effec¬tive coinci¬dental with appe¬arance of slight edema of the lower legs <cite>5</cite><cite>6</cite><cite>15</cite>  suggests that head-up induced volume shifts with an increased content of the extracellular fluid volume in the lower extremities play an important mechanistic role. The lower leg edema is thought to result in diminished pooling of venous blood in the legs and thereby in decreasing the excessive fall in stroke volume and cardiac output during a stand-up in the early morning <cite>5</cite><cite>6</cite><cite>15</cite>.