I take 100mg sertraline daily. is it okay to take melatonin supplements to aid sleep whilst flying to other side of world?

Absolutely OK: Melatonin is a natural hormone produced by the pineal gland. Melatonin levels decline predictably with age. Melatonin supplementation has a wide range of beneficial effects--not just as a sleep aid. It is a "regulator of regulators," and helps maintain circadian rhythms of other hormones, as well. Doses as high as 500 mg/day have been taken for prolonged periods without adverse effects.