It depends on the size of your seedlings not necessarily the light.

If they are large plants almost ready to be set out then you could start to harden them off by setting them outside for periods and overcast days are a good start. Do it gradually though.

If they are tiny newly sprouted small seedlings they are better off under the lights. They won't have to cope with wind and other things outside

They've already been outside for the last several days. They are about 4" tall. Now that I've potted up, I just don't have room anymore to keep them all under lights. They are having to "take turns" under the lights. So if I can get some of them outside on sunny days, I'm hoping that will make up the difference. So today was a bit overcast and I was trying to figure out if they were better inside, or out.

If that's the case they will get all the light they need outside and be ready to plant out when there's no danger of frost. After all their real growing time will be outside

What plants have you got? Perhaps some of them are frost hardy and can be planted out before the frost free date.

They are better off outside. Even on an overcast day the light is better outside unless you are using some high powered lights. Once they have been hardened off there's no reason to take them back in unless you are expecting frost or temps lower than 40F for most tropicals.

I was hoping that would be the answer. : )

Most of them are tomatoes. (Peppers, basil and eggplant I still have in the house.) I have about 300 plants, and only 5 lights. Still can't plant for another 2 weeks.

It has been way too cold to leave anything out overnight, at this point. That's OK, though. I just needed a way to get them light. So it sounds like what I'm doing is going to work.

I also have a bunch of peanut seedlings that I'm growing for my (optimistic) neighbor.

When you bring them in, don't bring them into a heated room. It is better if you bring them to a unheated garage, e.g.; otherwise a process of "reverse hardening off" might occur.

When you bring them in, don't bring them into a heated room. It is better if you bring them to a unheated garage, e.g.; otherwise a process of "reverse hardening off" might occur.

It would take more than just overnight or even a couple days to do that.