its recommended to leave it on because that is also their help for heat in the water...but i have turned it off before over night...i'm guessing your hatch rate is going to suffer though...so i'd just try and get some sort of cover for it so you dont give up a large loss in hatchings
well try it without and see how your hatch rate is...if its good then go for it...if not then you'll need to figure something out...but i think its going to suffer quite a bit without the constant light
i have a hatchery and i never even put a light on it at all... just natural window light and such. had great hatch rate. having the light on just speeds up the hatching, since i have no light on mine it takes like 2-3 days for the brine to hatch.
during the day my bucket of shrimp gets a little bit of natural light but other then that they don't get crap and they all grow in a bucket of old dirty saltwater. i think "oh yes you need light for them to grow" is sorta like saying "your goldfish tank needs an airstone".
just a bit from joyce wilerkson in her book "Clownfishes"
"Inadequate aeration is a common reason why brine shrimp cysts fail to hatch...Darkness may be the culprit research papers indicate that bright light triggers the hatching process."
maybe its outdated now...when i had dwarves and hatched my BBS it was still what i was told and read as well.