If you're really interested, read the book Why We Sleep by Matthew Walker. The book references a ton of studies that show, definitively, getting less than 7 hours of sleep is hugely detrimental to your health.
The book actually talks about how psychiatric disorders cause insomnia which creates a feedback loop of getting less sleep, which amplifies the detrimental effects of the disorder, which in turn worsens the insomnia.
It does not show that at all, especially not "definitively". Matt Walker's career is based on fearmongering about sleep. The actual studies are quite mixed. Here's a recent one that showed little to no effect of short or long sleep lengths on cancer risk: https://www.ncbi.nlm.nih.gov/pubmed/30463535.
Feel free to read the book, but don't do so without strong skepticism. In particular, the book never actually attempts to answer the question in its title, other than to present some vague, unfalsifiable theories. It presents tons of information, but no answer to the question.
There were parts of the book that I was definitely skeptical of, and after doing some research, found were not supported by any studies that I could find. So I will agree that you should remain skeptical about some claims... Really, you should be skeptical of anything you hear.
But the fact that under sleeping has serious detrimental effects on your health is supported by many studies.
Off the top of your head is there similar data on getting too much sleep? It makes a lot of sense that (past a certain point) lack of sleep could be harmful, since going without sleep is eventually fatal. However this could easily be compatible with too much sleep being harmless.
From what I've read, yes, there are similar risks associated with getting too much sleep. But don't take it from me, I'm just an engineer that recently became interested in the topic. If you search Google Scholar, you will find a ton of studies that will help answer your question.
Also, like the poster above pointed out, there are parts of the book that aren't really supported by studies, at least not studies that I could find. Make sure you do your own research before accepting anything as fact.
The most useful aspect of the book to me was just emphasizing how important sleep is to all organisms. There's a reason we've evolved to spend a third of our existence sleeping.
The book actually talks about how psychiatric disorders cause insomnia which creates a feedback loop of getting less sleep, which amplifies the detrimental effects of the disorder, which in turn worsens the insomnia.