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

Totally agree with Reine. At this age, your toddler possibly is still not aware to the idea of fear. To add to her suggestions, you may observe your toddler's sleeping behaviour first. If you see something wrong or if he/she is anxious during nighttime, instead of putting a night light, try getting him/her pet fish in an aquarium with dim lights at night. This is because it could be a possibility that your child is not afraid of the dark rather afraid or anxious to be alone in his room. Getting him a companion such as pets (it can be a dog, too) will help them feel not so alone in the room. If you will get a pet fish in an aquarium, place it in the area not too close to his bed so the light from the aquarium wont disturb his/her sleep. This is because according to research, exposure to light during nighttime decreases the secretion of melatonin--hormone that promotes sleep. Hence, exposure to night time light can decrease the quality and duration of your child's sleep.