What type of light? and how close do you have them to it?

All lights are not the same. They have different output, a different spectrum of light, and different heat output so require different distances from the plants. We need the specific details on the light you are using to answer your question.

But the most common cause of seedlings shedding their cotyledons is over-watering and/or over-fertilization.

Dave

I bought a lower watt bulb because of money constraints.
Home Depot sells it: Philips Plant Light 50w. My floor lamp has a horizontal brass, tllt-able shade/cover. The light is probably 8" above the plants. Others have said should be closer (2-4") to plants/seedlings but that won't work with my lamp. I turn it off at bedtime but I do put some of them in the windows when its sunny. I think I may have over-watered the Jiffy Pots 3 when I put a little water in the bottom of their dishpan container but I've done that before with no problems so I thought maybe it was the grow light. I suppose I did over-water them so trying now to dry them out a little.

Yeah that particular light, and that far away, couldn't hurt them. It does need to be closer (4" if possible) and if you can't lower the lamp then raise the plants. Set the tray up on box or something.

The Jiffy pots have a reputation for watering problems. They get too wet and then wick the water out of the soil and dry out. Once dried out they are hard to get wet enough again so the plants get over-watered. Any plastic container works much better. Plus unless you use a really good soil-less potting mix in them the soil compacts and smothers the roots.

Dave