Are toddlers scared of the dark? My 18 month old sleeps alone in his room. Should I be leaving the light on?

Fear, to me, is a social construct. If a child isn't exposed to elements of the fear of darkness (like scary stories about what creeps around in the dark), then he or she will most likely not even feel fear. At a toddler's age, reckon he is unlikely to be exposed to such elements yet, his imagination will probably be piqued at an older age e.g. 4-5 (when he is in school, interacting with friends, comprehending movies etc). That being said, agree with the other parents -- iyou can get a night light to alleviate your child's fears, if any. If not, then just leave it be.