Air pollution may be the cause of poor sleep, say the researchers from the University of Washington.
The results of their research are presented at the annual conference of the American thoracic society.
Scientists conducted an experiment for five years. During this time they took air samples near the houses of 1863 the volunteers of the six cities.
In the study participants before bedtime wore special devices that recorded body movement, thus determining the sleeping party or not.
The results showed that the higher was the concentration of nitrogen oxide and small particles in the air in the house, the worse the subjects slept.
The study authors attributed this effect with irritation, which causes polluted air. In addition, it can provoke breathing problems.
However, the high level of pollution was associated with a longer waking while waking up during sleep.
Scientists note that these results were quite expected: previous studies have shown that nitric oxide affects physiological and biochemical functions of the body.
