The text discusses the role of an enzyme called SR in the synthesis of two forms of serine, L-serine and D-serine, which are involved in sleep regulation. The study found that SR is the only known enzyme responsible for D-serine synthesis in vivo and that it can convert L-serine and D-serine reciprocally. The researchers tested the effects of L-serine and D-serine on sleep defects in mutant flies and found that only D-serine, but not L-serine, could rescue the sleep defect of srko mutants. Additionally, they found that D-serine, but not L-serine, was important for nighttime sleep duration and arousal response in srko mutants. These results suggest that D-serine plays a critical role in sleep regulation.