Lettuce seedlings seeming to close up and fold inwards

I've got a pretty basic (and likely suboptimal) seed starting setup going on and have some lettuces started in a seed flat bin (with a heating pad underneath) along with chives, basil, and jalapenos. As you can see in the picture, they're leaning a little bit towards the direction of sunlight (expected due to low amount of daylight currently) but they're also closing up and folding inwards on themselves (that's the best way I can describe it). Any ideas what would cause this kind of behavior out of a seedling?

I'm in USDA 8a in the U.S. South and hoping to push the start of the growing season by transplanting into cold frames in a month or two. I know that I may run into some issues with the amount of light available this time of year but I really don't want to have to buy grow lights so this is a little bit of an experiment to see how these seedlings do so early in the season with only natural sunlight. They sit in front of a south-facing window and I'd estimate they're getting about 7-8 hours of direct sunlight right now.

Hello Jeff!
Welcome to permies!

Good for you for getting started early!   They look pretty ok to me so far.   Short days can be a real problem though and I have not had great luck trying to cheat the system.  
I have used mirrors to boost light.  
Is there a way to put a mirror behind them so you can shine the sun back on them?  

Thanks for the welcome Samantha. Glad to hear that they don't look too bad. I've been honestly fairly impressed how quickly they've grown in about 2 weeks under these conditions. I am also supplementing with a very diluted organic kelp fertilizer every few days to add some extra nutrition.

I will look into adding an additional mirror as well although I would like to see how they do in sub-optimal "natural" conditions (although they are getting a heating pad underneath).

Would you recommend thinning to one plant per cube around now?

This might have something to do with which variety of lettuce you are growing.

It's a Bibb lettuce variety from Southern Exposure (so hopefully well-suited for the hot weather in 8a).

The seedlings have seemed to calmed down a little bit and look a little bit normal. I wonder if they were particularly nutrient-hungry in the first few days. I don't think there's been any big changes in light quality or amount since the folding phenomenon--if anything, it's been cloudier. I'll keep updated on how the other seedling species perform in these sub-optimal conditions. Maybe I should make a new more general thread: "Starting seedlings in natural winter light for cold frame transplant" or something similarly named

I'm a few weeks later with the seedlings and they're doing ok. We've had daytime temps in the 70s with full sun and nighttime temps in the 50s so I've started hardening them off outside for a few hours a day. The stems on all the seedlings are pretty weak, but I assume this is due to the low-quality light they've been getting inside the house. Maybe some actual sunlight in the next few weeks will help them strengthen and grow faster. I would flip the seed tray by 180 degrees every few days to "flip" the direction the plants were leaning to, and maybe that is also a culprit for the thin stems.

The lettuces look good in color but can barely stand upright on their own. In a few of them, the small leaves towards the bottom have yellowed and died, and I'm not sure what's causing that. I mix a capful of kelp fertilizer with about 3 cups of water and feed that once a week. Could they be getting too much nitrogen? The soil is pretty much always damp and I'm on top of watering generally (besides a few days where I left for an extended weekend and had to "overwater" a bit so that they wouldn't dry out).