It depends on the person, in my opinion. If someone has allergies, then no, it isn't healthy for your dog to sleep with you but many people let them anyway (myself included...but my allergies are mild and I don't react much to longhaired dogs so I'm fine). They can also disrupt someone's sleep if the person moves around a lot and wakes up easily because the dog or cat is in the way. This is all just what I've heard and is probably true, but I know I sleep much better with my dog than without.