7 days post tummy tuck, is it ok to resume melatonin for sleep?

You should be perfectly fine to resume taking melatonin, it's always best to ask the surgeon who performed your surgery because everyone has different post op protocol, although I see no reason why it should be a problem!

Resuming melatonin should be no problem, but please check with your surgeon.  I'd avoid taking any medications that could oversedate you if taken with the melatonin.

I am unaware of any surgical side effect as far as bleeding risk or wound healing problems associated with Melatonin. It should be ok to resume its use after 7 days. I would caution you on the interaction of narcotic pain medications with melatonin. Sometime combing drugs with sedative effects can be dangerous. Since you are not taking any pain meds...your risk for any problems seems very very low if you have any risk at all. 

It is great that you are asking this question. It is likely that your plastic surgeon will okay melatonin. But it is always wise to check on all supplements, herbal remedies, homeopathic remedies and of course prescriptions both before and after surgery. 

Melatonin should be safe to resume one week after surgery.  If you have medication questions, you can always ask the phramacist who should know all of your current medications.