Natural sunlight is definately good for turtles, for sure... but I don't think you should rely only on that. For a few different reasons.
1) Draft.. if the window is open and there is a draft your turtle could have respiratory problems later.
2) Turtles should have access to UV and basking light for at least 10-12 hours a day, every day. They won't get the UV rays through the window if it's closed and I'm not sure that the sun would be coming in that open window for that much of the day.
3) It would be much harder to regulate the temperatures with that setup, as opposed to a setup with lamps.
I agree that it's awesome for them to have natural sunlight when possible but I think they'd thrive more from a constant setup with regulated temperatures and no drafts.


