Welcome to the Helpful Gardener,
I'm thinking that you probably also shocked them initially when you planted the seeds next to the window. Temperatures beside windows rise and fall drastically between day and night and during weather extremes.
You said that you have "white lights" I'm assuming that white lights are not grow lights. I will recommnend that you purchase some grow lights, they come in flourescent bulbs, incadescent bulbs and these conical shaped bulbs. With my experience the conical shaped bulbs don't work because they die on me in a few days.
Anyway, my point is that with the proper bulb, your lettuce will recieve the proper wavelengths of light that it requires to carry out photosynthesis.
The photoperiod (length of time the lights are on) shouldn't cause you any problems. Lettuces are an annual and generally grow during the spring and summer when the photoperiod is quite long.
Don't wait until the soil is dry when watering. Just keep it moist but not wet. The easiest way to figure this out is to insert a toothpick or chopstick into the soil and pull it out. If the dirt sticks to the stick, don't water. But, I just feel the soil with my finger and if it is starting to feel dry, I water.
Good luck with your lettuce!
As always, any other advice is most welcome!