Sentence:  Irregular bedtimes may disrupt healthy brain development in young children, according to a study of intelligence and sleeping habits.  &para; Going to bed at a different time each <WSD>night</WSD> affected girls more than boys, but both fared worse on mental tasks than children who had a set bedtime, researchers found. 
Meaning of the word marked with <WSD> tags:  The period of darkness beginning at the end of evening astronomical twilight when the sun is 18 degrees below the horizon, and ending at the beginning of morning astronomical twilight.