How do you make work like a day at the beach?

Do it on your day at the beach.

I went to the beach yesterday - the day seemed so long, I got so much work done, and I was totally relaxed and enjoying myself the whole time. Today, I spent about the last hour and a half in the library studying, and I'm spent. I'm wondering where my day went, and how I was so much more productive yesterday. It's all about a state of mind, and this here is a world of pain.