What would an employer think if an employee agrees to work 15 hours a day for 6 days a week in the management level?
How the hell can someone manage effectively with that many hours?!
Those are the hours an individual contributor would pull for a limited-time crash project.
I'm not an employer or manager, but my immediate thought would be that this person is unsuited for management because they don't know their own physiological limitations (unless they have previously divulged, and demonstrated, that they are one of the extraordinarily rare people (~1% - A tiny percentage of the population needs only 4 hours of sleep per night) who can function at peak with limited hours of sleep.