Timing of spraying

When spraying for spider mites (I've currently got a home cooked up spray consisting of habanero peppers, soap, and vegetable oil), I've heard to only do it when the lights are off or an hour or so before coming on. Any reasons for this? Can I do it with the lights on? Any adverse effects?

The water magnifies the light and burns the leaves. At least that's what I've always been told.

I spray with lights on, however I have a fan running across the plants at all times (across the top of a scrog) - I've never had burn issues... maybe just raise your light a little if your concerned...

Are your plant(s) in pots?