I am now prepared to publish my observations regarding the relative consequences of getting insufficient sleep versus getting just enough sleep.
If I have slept well enough, I find that not only is my initial energy level better, but so is the recharge rate after physical exertion, to the effect that the subjective length of the day is much longer and I am able to get more things done.
By contrast, on days when my sleep has been interrupted — or if I’ve been unable to get much sleep at all, interrupted or not — I start the day tired, get more tired with less exertion, and take much longer before I am able to resume even minimal activity. The day is subjectively much shorter and I get almost nothing done.
I have no conclusions as yet about oversleeping, though I am inclined toward the prejudice based on rare experience, that these incidences result from difficulty getting to sleep for an extended period of time after retiring, and this may account for the feelings of grogginess and low motivation after rising later in the day. The apparent subjective length of such days may also be truncated as much due to the loss of morning activity hours as to a lack of energy or stamina.
So, you don’t need to fly at or near the speed of light to manipulate the rate at which time passes. You only need to cripple your metabolism with insufficient sleep.