I just read this again, and I thought of your post.

http://ratguide.com/breeding/pregnan...itter_loss.php
Light
Cycles of light and darkness are also important in breeding rats, and they should be provided with 12 hours of light and 12 hours of darkness. Deviating from this cycle can affect the reproductive process. Duration and type of lighting can also affect estrus behavior. Rats exposed to noise and flashing lights during pregnancy had an increased absorption rate and variation in fetal weights. Subtle variations in temperature and humidity have been shown to cause decreased reproductive efficiency. Proper ventilation can have an impact, reducing the spread of disease. It also provides a stable temperature and lessens the chance of drafts.