REST

"It is a common experience that a problem difficult at night is resolved in the morning after the committee of sleep has worked on it."

-- John Steinbeck, Author

Our bodies, as well as our minds, need to rest.  Studies have shown that a good amount a sleep does wonders for both the body and the brain.  Never neglect the importance of rest, no matter how hard you are working towards achieving a goal.  At the same token, don't indulge in it too much as you won't achieve what you are striving.