Yeah, whilst I get home work from my school and study 8 hours.. fatigue happens to take place. Once, I had law exam in grade 11 and I was studying the whole night, I didn't look at the clock and when I looked and the number count at the bottom.. I must go to bed... it was 4'o clock in the morning. So, yes after being horrendously busy, I was tired and couldn't sleep.
Now the question here arises: why is that? The answer is simple, once you are determined to do something, you don't care about your comforts or your sleep, you only care about your work which is to be done. When one finishes his/her work, then he/she seeks the pleasures of slumber. It is rightly said, "to achieve something, sometimes you have to lose something"... here, sleep fits in this case.