If a normal-weight person undergoes significant long-term sleep deprivation -- like >50% of nights on 4-6 hours of sleep over the course of decades -- what would the expected effect to be? And if the person's bloodwork nevertheless shows no particular signs of poor insulin response?
Well, you're getting into measuring sleep by time, which is an antiquated idea that we need to move on from. I write about this on the affectable sleep blog [1]. We shouldn't be looking at time, but the restorative function of sleep, and we need to take each individuals restorative function as independent. The average diet recommends 2400 calories (I think), but you wouldn't say that if someone got only 2000 calories a day, they'd starve.
However, on a population level, it is known that shift workers have increased incidence of diabetes, heart disease, obesity, etc. Some of that likely comes down to socio-economic factors, but we can't ignore that daytime sleep or potentially even shifting schedules, is not as restorative as consistent sleep, regardless of sleep time.
I feel I haven't answered your question, but I think this is the best I've got.
Asking for a friend.