Impatiens need light for germination. I start all my seeds under lights, and with a few exceptions, and with a few exceptions place them under lights as soon as they are sown. Parks Seed catalog index tells what seeds require light and those that require darkness for germination. Unless it is a seed that requires darkness I start it under lights.

Lighting hours per day can be as little as 12 for shade loving plants to as much as 24 hours per day. If a plant starts to get tall and leggy it's an indication it is in need of more light and is reaching for the light. There are of course exceptions to this rule. One for example is Gloxinias. If they get more than 15 hours per day seedlings will prematurley start to form tubers and will die off. Unless you have a very brightly lit room to put them in mot annuals will need supplement light until transplant time.

You can view some of my seedlings at the link shown below.

Marv - Montezuma, IA

Here is a link that might be useful: Seedlings etc.