While most well-known as a natural sleep regulator, melatonin also has many other important functions.3 Notably, it plays an important role in cancer prevention4 and may prevent or improve certain autoimmune diseases, such as Type 1 diabetes.5

It also has anticonvulsant and antiexcitotoxic properties,6 and is a potent antioxidant7 with the rare ability to enter your mitochondria,8 where it helps prevent mitochondrial impairment, energy failure and the death of mitochondria damaged by oxidation.9 It also:

Boosts immune function

Helps recharge glutathione10 (and glutathione deficiency has been linked to COVID-19 severity)
May improve the treatment of certain bacterial diseases, including tuberculosis11
Helps regulate gene expression via a series of enzymes12
As noted in the Journal of Critical Care:13

"Melatonin is a versatile molecule … Melatonin plays an important physiologic role in sleep and circadian rhythm regulation, immunoregulation, antioxidant and mitochondrial-protective functions, reproductive control, and regulation of mood. Melatonin has also been reported as effective in combating various bacterial and viral infections."