Most of us would love to have a short nap in the afternoon, right? But that would be frowned upon at work. However, a new sleep study found a 20-minute power nap actually improved our reaction time by up to a third. Thinking of trying your own test perhaps this weekend? Heads-up in 20 minutes, as sleeping longer leads to a deep sleep that will leave you feeling groggy and more likely to be cranky when you wake-up. NOTE: Teens to babies did not share these results. As their brains are still developing they require more sleep.