As we all already know that anything that has positive sides also do have negatives; same goes for sleeping. Sleep is healthy to one's health when done appropriately and it can be unhealthy too if not done rightly. You can't be awake while you are meant to be sleeping after you're tired. And you can't be sleeping when you are suppose to be awake when you are not tired.
Too much of sleeping is harmful to one's health same with insufficient amount of it. Sleep that is not done sufficiently can lead to health related issues like chronic headaches, obesity, backache, just to name a few.
Meanwhile if you are over indulging in sleeping too, you can become exposed to overweight, diabetes, high blood pressure, etc.
Notably, the recommended hour for sleeping correctly is 6-8 hours. Sleeping above this hour is dangerous to one's health and sleeping below it is harmful too.