starting seeds in sunroom in Canada

I have an uninsulated sunroom which I plan to use to start seedlings.  I'm almost finished making a set of shelves encased in wire mesh to keep the cat out.  The seedling shelf is set against the windows and will get sun from about 10am to 5pm.  In about two weeks I plan to start half my seedlings and germinate them in the sunroom at this point it'll still be below freezing at night.  So the dilemma is how to keep the seedlings warm overnight until the room warms up in the morning?  I could put in a small electric heater to heat the whole room, but maybe there's something more energy efficient.  Any ideas?  Thanks.

Is it possible to have the set-up be a couple of inches away from the glass, so you can pull a blanket as a curtain over it in the evening? You could leave it open towards the house and the warmer spaces, and only shield it from the cold glass and dark night.

Many plants will sprout and grow even when it freezes mildly at night. In my experience, most of the greens are fine starting in cold weather, such as lettuce, chard, kale, spinach, arugula, claytonia, dill, fennel, parsley, also onions... It's only those intensely warm loving things that can't take it, like tomatoes, peppers, and their family, most of the squash and melon family, basil... I think. I may have gotten one or two wrong. I grow greens and herbs all winter in a solar greenhouse that drops below freezing for nights in Dec, Jan and Feb, and they thrive.

As above it depends what seedlings you have but I would be wrapping something around your shelf (plastic sheet, blanket, mylar, tinfoil)to keep the heat in and put an incandescent light bulb at the bottom.  If not that then yes an electric heater.

You could put a seedling heating pad underneath, which might be cheaper than heating all that air.

Even more permie would be to put some rocks or some other thermal mass in a sunny part of the sunroom to absorb heat during the day and then radiate it at night to mitigate temperature fluctuations.

I think a heating mat under the seedlings, as Chip mentioned, may be the best way to go. Not necessarily the mats marketed and sold for that purpose (but they do work fine) but maybe you have a heating pad already. I use a heating pad set on low to encourage germination of certain varieties (like peppers) which prefer a warmer soil temp around 75-80 to germinate. You didn't mention what you were starting the seeds in, but a cell tray with a clear plastic dome lid might be ideal, as the dome would help trap the heat from below and prevent a lot of heat loss. With this method, you're only heating what needs heating which uses much less energy as opposed to heating the entire room. Hope this helps!

Thanks everyone for the suggestions.  I think I will pull the shelves out from the windows a bit and wrap curtains around.  And then with an incandescent light on the floor that would probably keep the soil just above freezing.  Then after a couple of weeks it'll probably e warm enough overnight.  Luckily I'm not planting any tomatoes, my husband is avoiding high histamine foods.  I think my new setup will be much easier than last year, when I kept bringing the seedlings in the house at night, and I had a heck of a time barricading the cat out.