I can tell when I let myself get sleep-deprived, I am much less mentally resilient. I make more mistakes. I have a harder time solving problems.
Most of the time I would get more done in the long run if I went to bed at a reasonable hour and got a good night's sleep. It's probably my worst habit with regard to my health -- mental and physical.
This may be the scariest part to me:
What may be the most critical is the fact that scientists have linked sleep to learning and fixation of memories in the brain, among some other nervous functions, underlining its importance further.
Also, we can go in the hole:
One thing you should remember is that sleep is cumulative. What this means is that if you fall behind on sleep during one night because you’re busy with school or work, you will have a sleep “debt” that your body has to cover. Until you do, you might feel tired and unable to concentrate.
My "sleep debt" is about like the national debt. I probably won't live longer enough to pay it off. Unless I drink more coffee.