So I'm in the Uni now and mostly during examination week when we're busily preparing for a paper, I can only go so much as 1:30 min and then nothing enters the head again. So to be able to continue learning, I have to take a nap. I normally won't be able to sleep within that time frame since it's still day time. However I do get to the first stages of sleep where I'm conscious but inactive. When I wake I'm good to go again. A nap is totally effective and beneficial.