Melatonin is a hormone in the body generated by the pineal gland in the brain. It can also be found in other areas like...
Some people report vertigo, lightheadedness, or mild dizziness after taking melatonin. They may feel like the surrounding space is spinning due to a sense of imbalance. In many cases, this may be associated with the risk of developing benign position vertigo after taking melatonin. This condition may affect the canals of your inner ear and cause a spinning sensation. Make sure to consult your doctor as soon as you feel this symptom to get the best advice. 
Upset stomach, cramps, diarrhea, constipation, and nausea are all common side effects of taking melatonin. This is not unusual because melatonin may irritate the GI system. Therefore, it is advisable not to purchase this supplement in bulk. Make sure to test how the body reacts first before continuing. Also, the effects can vary between different brands or types of melatonin supplements.