Melatonin-d4 is primarily used as an internal standard in analytical chemistry for the quantification of melatonin in various biological samples. Its deuterated structure allows for precise and accurate measurement in mass spectrometry, ensuring reliable results in pharmacokinetic studies and clinical research. It is also employed in the development and validation of bioanalytical methods, aiding in the detection of melatonin levels in plasma, serum, and other matrices. Additionally, it plays a role in studying melatonin's metabolism, distribution, and excretion in the human body, contributing to advancements in sleep disorder research and therapeutic monitoring.