Growing in winter (hoop house)

So I have a 16x24' hoop house with 6mil plastic sheet covers. It's not bad for extending the season but sustained daytime temps <40 and night lows <30 make it difficult to keep warm right now.

I'm wondering if anyone has had success growing in those sort of conditions - growing anything, really. I'm thinking that my best bet would be to use heating cables in the beds and put "sub-hoops" over them (basically a mini hoop-house over the beds, to give a second layer of containment/insulation.

Trying to figure out what I might be able to do with that sort of an arrangement. My thinking is that keeping the soil warm is the most important part, and that the soil warmth will help warm the smaller airspace above the bed too.

That's pretty darn chilly and a big space. Perhaps with raised beds and cables or tubing more options would be available. I'd be leery of sub-houses reducing the already weaker sunlight. Winter greens are something that might work well, though they aren't the most exciting thing to grow.

Yeah the light is definitely another concern. Definitely would be raised beds.