Here's how I solve the problem of leaving baby peppers in the window sill propogator. Not an LED grow bulb in sight!
Firstly use recycled card behind the propagator. This one is heated at the base.
Cover it in kitchen foil to reflect the light.
This means the seedlings do not grow forward towards the window but grow straighter, avoiding turning which makes plants grow leggy and weak.
Once under way, plants can also be moved from the toasty bottom heated south facing sill to the cooler west facing one, which should help to stop them over-growing too soon. The next stage (from propagator to greenhouse or polytunnel) is always the trickiest!
Heat a Greenhouse with Compost and Manure by Maddy Harland
Book: Edible Paradise