How harsh is prison in North Korea?

There is no doubt that North Korea treats its prisoners harshly.
When outsiders are arrested, they are often sentenced to hard labour, and that's exactly what it is - compounded by the severe oppression of isolation and helplessness.
The BBC knows of one former prisoner who was broken psychologically by his treatment. Many years later, he remains too traumatised to talk about it easily.
But others have described their experience in detail.
In December 2012, North Korea charged missionary Kenneth Bae with acts "hostile to the republic".
He had visited the country many times, but was stopped on this occasion and a hard drive with Christian material was discovered.
For this "crime", he was sentenced to 15 years hard labour, and only released when his health deteriorated seriously - just as seems to have happened in the current case of Otto Warmbier.
After his release, Mr Bae wrote a memoir, "Not Forgotten: The True Story of My Imprisonment in North Korea" in which he said that he was interrogated from 08:00 in the morning until 22:00 or 23:00 at night every day for the first four weeks of his imprisonment.

Source: BBC News