The Humboldt line is meant to be sprayed at any time during the light cycle and then have the lights raised or switched to supplemental lighting (I use a 24" t5 fixture for this exact purpose) for the 2-4 days following application. It's not about whether it is physically applied in dark or light, but how it changes the plant's response to available light and fertilizer while it is still in the plant's system being metabolized. I spray my Gravity and SnowStorm during 'day' but run the backup lighting (~150w of flouros I think) instead of my 2k of HIDs for the following 48 hours if it is a heavy application. The SnowStorm (aka purple maxx) alone, mixed weak, I just spray it approximately whenever I remember to, light cycle aside, and have not yet seen any problems from it. I run about 5mL in a quart sprayer for that one and mist it on REAL fine.