I have many issues with my sleep cycle and quality of sleep. Melatonin is one of the few things that alleviates my problems. However, for me, it's only good for one night. I can take melatonin for one night out of a month, and get a blissful, restful night of sleep. If I take it two nights in a row, evil side-effects start creeping up.
It begins with a headache. If I take melatonin two nights in a row, the next day I will wake up with a headache that's hard to shake. My dreams, while very vivid and pleasant the first night, become more gloomy and depressing the second night. By the third day, my dreams are full-on, vivid, terrifying nightmares. I feel depressed and angry all day, and the headache borders a migraine. After four days of melatonin, I'm a wreck. It no longer puts me to sleep, nor gives me restful sleep. It becomes nothing more than a pill full of nightmares, depression, anger, and a bad migraine.
The first time I started taking melatonin, I had no idea it was causing these side effects. It took a while to pin down, and I just assumed it was related to my ongoing battle with sleep. I do recommend it, just know that you may have adverse reactions to continued use.