In bipolar not sleeping equals being sectioned and put in a mental ward where one of the first things they will do is feed you sleeping meds. Sleep is absolutely essential for a bipolar sufferer.
I am currently on diazepam. Started on 25 mg as I went over the maximum dose of zopiclone. I am down to 5 mg now. I found it quite easy to drop to 10 mg, then got stuck there for a while, and then at some point around two weeks ago I moved down to 5 mg in one step and now I have nights where 4 mg is enough and sometimes I have to go up to 5 or 6. I am glad I managed to reduce the dosage so much -- I now have dreams again, and it's much easier to get out of bed in the morning.
If melatonin works for you I absolutely recommend you stick with it. It is much, much less dangerous/addictive than prescription meds like zopiclone, ambien or benzos. If it puts you to sleep -- more power to ya! (Also I'm a bit jealous

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