A study published in the journal Microorganisms showed that melatonin, popularly known as the "sleep hormone", can worsen the inflammation of the intestine and impair the action of the gut microbiota.

The community of bacteria and other microorganisms in the gut is beneficial to health, as it aids digestion and boosts the immune system. Additionally, weight gain, high blood sugar, above-normal cholesterol, and other illnesses can be exacerbated by an imbalance of toxic and beneficial microorganisms in the gut.

The study involved an experiment in which colitis was induced in test subjects and they were treated with melatonin. During the study period, it was observed that their condition became worse instead of improving.

Hence, the researchers concluded that the negative effect of melatonin depends on the bacteria that live in the intestine. They also noted that it is also associated with inflammatory diseases in the region. Additionally, they also noted that certain features of the gut microbiota increased inflammation and dysregulated the immune system in response to treatment with melatonin. Thus, damaging the digestive system. 

(Source:https://www.business-standard.com/health/study-finds-common-sleep-hormone-pill-can-worsen-bowel-inflammation-123060400653_1.html )