Insomniacs may not only toss and turn all night - their symptoms may be worsened by a constant need to get up and urinate, Danish researchers reported on Wednesday.

Tests on 20 people who were deliberately deprived of sleep produced more urine at night, and it was saltier than usual, the researchers said. The effects were much stronger in men than in women.

Birgitte Mahler and colleagues at Aarhus University Hospital in Denmark found no differences in urine production or quality during the day in their sleep-deprived volunteers.

But things went awry at night, they told a meeting sponsored by the American Physiological Society in Austin, Texas.

Sleep deprivation reduced the usual dip in blood pressure that is seen at night, they found. This, in turn affected blood pressure-related substances such as rennin, angiotensin II and aldosterone, which could have in turn kept the kidneys on a daytime schedule, the researchers said.

The urine of the sleep-deprived volunteers also contained higher than usual levels of sodium and potassium.