Best time to take Sertraline?

This antidepressant, like most SSRIs may cause insomnia, talk to your Dr. if it affects your sleep and take it early in the morning, and for other patients it may cause somnolence and dizziness, again talk to your Dr. and take it at night.
Everyone reacts differently to medications, so you will have to first take it as prescribed by your DR., see how it goes and take it from there, always letting your prescriber know.
It is relevant is that you take it at the same time everyday, this is so you have an even amount of the medication throughout 24 hours (although Sertraline is not ER-extended release) until your next dose.
Always maintain your Dr. informed.