Hm, I don't know if plants take in nutrients at night, while growing (plants only grow at night), or if they save it up during the day to grow at night?
Hm...
Hopefully someone will know.
But I really don't think it matters too much.
its best to dose in soon as the lights are on or just before.
its like watering the lawn before the sun comes up, its the best time.
i sometimes (dry ferts) dose mine about 2-4 hours after the lights kick on but its no big deal in my opinion, the plants still grow fine so i wouldnt worry to much.
if your dosin excel i would say when lights are on or just before.
excel only last 24hr i think? maybe 12?!? in water so its best to dose it asap i would think.