"P.V.T. is tedious but simple if you’ve been sleeping well. It measures the sustained attention that is vital for pilots, truck drivers, astronauts. Attention is also key for focusing during long meetings; for reading a paragraph just once."

I can't say how true that is when you're doing a ton of email all day or research, and its more a function of the sum total of small delays in re-reading that build up over the course of an entire day. Contrasted with a day when your mind is so clear that you require no re-reading, it feels like you are so much more productive.

Additional sleep links:

1. Sleep: How to hack your brain (http://www.dustincurtis.com/sleep.html)

2. The 4-Hour Body: Engineering better sleep (http://techcocktail.com/book-preview-the-4-hour-body-enginee...)