Modern LED lights are great, but definitely quite powerful! Pro-tip? We recommend using a free “light meter” app on your phone to dial in lighting to about 20,000 lux at the plant’s tips. This is still about 2x what CFL lights can do right below the bulbs, but still quite safe for seedlings! Above 25,000 to 30,000 lux and plants can get sun burn!

But, to your question. If you still have decent greenery on your plants, it should still be safe to harden off. I’d recommend being a bit more careful and maybe spend 2-3 extras days at very low time exposures, like 1-2 hours. This will help mitigate the fact your safety margin is not as good as it usually is. But, if your plants are severely sun burned (err, LED burned), then you should wait for a little bit of greenery to return before you start hardening off. With tomato seedlings, this shouldn’t take more than a week or two. Good luck and let us know if you have any questions!