Melatonin is a neurohormone secreted by the pineal gland that helps regulate sleep. It is the reason why people tell you not to go to sleep watching TV or with any electrical devices as it appears to be sensitive to light therapy. And therefore the primary reason why people use melatonin is to normalise sleep patterns.
If you are a frequent flyer and suffer with jet lag, or work shift patterns or just general long and unsociable hours I would strongly recommend you trying Melatonin if you want to keep improving your fat loss + muscle building efforts as well as being productive throughout the day.
I would recommend taking 1 tablet 30-45 minutes before bed and you should find that you are falling in to a much deeper sleep.