alt Hacker NewsI would suggest looking into the medical research on the downsides of getting little sleep. After a single night of low sleep (and 4 hours is low), even though you'll feel fine, your body will not process a lot of things the way it normally should (e.g. insulin response is significantly degraded).
Over time, it supposedly significantly increases the likelihood of certain diseases/conditions.
Peter Attia was one of those people who got by with little sleep, and for years, well into his medical career, was dismissive of those who preached the importance of 8 hours of sleep a day. He then looked into the research, and completely changed his mind:
When I was looking into the dual n back stuff a while back, I remember reading one report where the person would feel subjectively fine when sleep deprived, but his nback score would go way down.
In other words it's possible to "feel fine" on little sleep and yet be significantly cognitively impaired. Worth measuring that, if possible.
(It might have been Gwern, he's got a big page on the subject.)