In a study on fruit fly behavior, researchers used video recording and analysis to measure sleep in both wild-type and mutant flies. Under a 12-hour light/12-hour dark cycle (LD condition), the mutant flies (shmt-es) had significantly shorter durations of both nighttime and daytime sleep compared to wild-type flies. To determine whether the sleep defect was caused by a lack of either L-Ser or D-Ser, the researchers rescued the mutant flies with either L-Ser or D-Ser and found that feeding either substance could restore normal sleep patterns. Therefore, the sleep defect in the shmt-es flies may be due to a deficiency in either L-Ser or D-Ser.