Seedlings flop over four days after sprouting under a shop light Seed Starting

Two separate things are happening and they need different fixes. The stretching is light: 30cm above the tray with an ordinary shop light is far too far away, and seedlings reach for it. Drop it to 5 to 8cm above the leaves and raise it as they grow. The collapse at the soil line is damping off, a fungal problem that thrives in exactly what you are giving it, a closed humidity dome over damp compost with no air movement. Take the dome off the moment the first seedlings emerge, not when they are established, and put a small fan on the tray for a few hours a day.

The fan does more than prevent damping off, it also thickens the stems. Seedlings that get a bit of air movement or a daily brush with your hand come out stocky and transplant far better than protected ones.

Fourteen hours is fine, sixteen is better for tomatoes, but hours cannot compensate for distance. Light intensity falls off sharply with distance, so a lamp at 8cm for 12 hours beats one at 30cm for 16 every time.

Once damping off has appeared in a tray I bin the compost rather than reuse it, and wash the tray. It persists, and starting again in clean sterile mix is cheaper than losing a third sowing.