Question about taking melatonin?

If you take melatonin to go to sleep (which I do, and it totally helps – about 6mgs/night) but wake up in the middle of the night and can’t fall back asleep… is it safe to take more?

I had surgery yesterday and just woke up… not in pain but on meds, so can’t take any sort of sleeping pill. Would just like to be able to sleep – I’m horribly bored!

Any input? Exercise is not an option… I’m trying to read and listen to soothing music until I get tired again.