6-Hydroxymelatonin (6-OHM) is a naturally occurring, endogenous, major active metabolite of melatonin.

[5][6] The determination of 6-OHM in human urine has been used to track the metabolism and excretion of melatonin using LC-MS/MS, providing quantifiable insights into circadian rhythm regulation and its oxidative role as a biomarker.

[7] 6-OHM is one of four of the primary metabolic products of melatonin in the liver and is also a byproduct of its breakdown due to exposure to light.

It is known to be very effective in protecting cells from oxidative damage caused by ultraviolet (UV) radiation.

[8] Based on comparisons with other melatonin-related compounds, it is suggested that the protective effects of 6-OHM in mitigating oxidative stress are primarily attributed to their ability to scavenge free radicals.