I'd expect the optimum for mortality and congnitide ability in this case would be about the same. Else we'd have the case of something worsening your health but making you smarter (whereas for food, exercize, etc. it's the inverse).

>I need at least 8 and preferably 10 hours of sleep (as an average).

While there is more or less sleep required by different people, note that what an individual "needs" might not be the optimum for them under all conditions, but be such because e.g. they are stressed, overworked, sleep badly, etc.

E.g. someone with apnea (e.g. due to weight) might need 10 hours of sleep to feel well, but that is because they have low quality sleep, and need the extra time to conpensate - not because their organism, if cured of its apnea, actually needs that amount of time optimally.