Is too much sleep unhealthy

It depends on what you mean by "too much" sleep. If a person is sleeping more than 10-12 hours a day, then it could be an indication of illness or depression. Quite often, when a person is depressed, they will sleep upwards of 16 hours a day. If you, or someone you know is sleeping that much, you/they need to see a Doctor Who can diagnose and treat the cause of sleeping so much.