short version melatonin is a chemical naturally occuring in the body. it controls things like a persons internal clock (those things like telling you when its time to wake up or go to bed or eat or thirsty with out having to physically look at a clock) every human being has an instinctual internal clock. Suplimenting with Melatonin without blood tests backing up that someone's Melatonin levels are low can cause a person to be overdosing their body and throwing off their internal clock to where their internal clock is no longer telling them its time to sleep. this can cause a person to react in many ways like changes in ones sleep schedule or having trouble getting any kind of sleep.

to realign ones internal clock after sublimenting with Melatonin it takes going through a few to a week of not using it any more and going through a sort of withdraw from it with symptoms of anxiety, restlessness, lack of sleep.

when this happened to me I stocked up on some great books, and non medicated coping tools for my anxiety and sleeplessness like a warm bowl of hot cereal, microwaving a scented bag of rice or small beannie toy, calming music.... my point everyone has their own non medicated personal items that bring them comfort, for some its cooking scents or flowers or warm items, warm clothing, a favorite chair, favorite sheet set.....

my suggestion is contact your doctor and they can help you to get your internal clock reset so that you can have a better night sleep with out depending upon the extra melatonin. your doctor can also do a blood test to see what vitamins/ nutrients that you may be low on and prescribe yo what you need that can also help with your anxiety/sleep problems.