A lot of people suggest spraying it right before lights out but I've been using it during the light period w/ a 600w HID 16" away...haven't noticed any burning but I've also got a lot of airflow going through there n they dry quick. Might be safer to just spray before lights out. That stuff reaks like ass but it's really effective. Fungus gnats gone in 1 day for me with spraying neem and adding a layer of perlite on top of the soil