Why do we get a 'second wind' of energy at the end of the day?

Sometimes people get a sudden burst of energy, also called a second wind, before they try to go to sleep. (Image credit: urbazon via Getty Images)

After a long day, you'd expect to feel exhausted by bedtime. But sometimes, the opposite happens: Instead of winding down, you get a sudden burst of energy, making sleep feel frustratingly out of reach.

But what's behind this second wind?