I have this disorder and there is absolutely nothing "crockish" about it.
It's horrible and very real.
I've been a 'night person' since I was a little kid. My mother used to have to almost sit on me to get me to go to bed at night, and then she'd have to drag me out of bed to go to school in the morning.
Let's see ... it's about 2:41 AM. Lunchtime.
I don't know whether or not I have it, but I work midnight to noon, so it is no problem. On days off I sleep late, stay up late. No biggie. I have never worked banker's hours in my life.