I have issues with insomnia (waking up in the middle of night) but don't think I have circadian rhythm issues. Could melatonin help?

No. While melatonin could potentially help you sleep, it would likely be more beneficial for sleep onset insomnia rather than sleep maintenance. You may consider seeing a sleep specialist who can determine if you would be a candidate for cbti or if you require pharmacologic treatment with a longer acting hypnotic.
Probably not. Melatonin has some evidence supporting its use in jet lag and other sleep problems associated circadian rhythm disturbances. Based what i've read, and my clinical experience, melatonin rarely helps with insomnia.
Insomnia. If you are on no medications that could cause insomnia, and you are healthy physically, low dose melatonin (3mg.) might be initial trial option.