Hi i hope i got your problem right. your plants leaves are drooping/ wilting....... well i wont blame it to the LIght, either the rootzone. Sounds as if your roots have problems to breathe... check up the rootzone, look and smell into the container. If everything is ok , you shuld smell nothing and see white healthy roots, if you see browinsh slimy thigs your roots have a probem.....Iam pretty sure your plant can bare 18 hours of light