Superman a little jealous: micro-sleeping. Superheroes often struggle with finding a balance while trying to save the world, work a 9-5 job, and get enough sleep at the same time. For Wally West, he once revealed he's been able to take micro-sleeps while fighting crime at the same time.
Derived from the Speed Force, the Flash has many different powers which are much more than "he can run fast." Versions of the Flash have used their powers to travel through time, develop a healing factor, fly, and even decelerate their own aging. The Speed Force has become a MacGuffin of sorts, as it's been used to explain powers and abilities that drive the plot and ultimately help the Flash save the day. Wally West once showed he can fight crime and get some sleep at the same time - in an action scene that explains how the speedster is always so well-rested.
In The Flash 80-Page Giant #1 (1998) story "The Speed of Life," by Mark Millar, Ariel Olivetti, Tom McCraw, and Bob Lappan, the meta-story features the actual comic book writer Millar meeting with Wally West as he struggles to come up with his next big story. Wally suggests he write a 10-page story on the Flash's sleeping patterns, telling Millar that he usually is awake all night and takes micro-sleeps during the day. He's even been able to sleep while fighting, as he mentions he's gotten rest during the middle of some boring fights.
The Flash's ability to get some rest while he's fighting is likely envied by some of Wally's fellow Justice League , especially the Man of Steel. While Superman doesn't necessarily need sleep to function physically since he's powered by yellow sunlight, he does require a little bit of sleep to keep mentally sharp. Imagine if he could have a nap mid-fight like the Flash? He would be able to stay fully mentally prepared all the time - making DC's most powerful hero even stronger.
The Flash's speed makes him more than capable of doing multiple things at once, but taking a quick nap mid-fight just shows how amazing his powers truly are. He can knock out one of his opponents, have a quick nap, and take on someone else - all in the spans of microseconds. The well-rested hero's secret power hasn't been featured heavily, but Wally West's Flash can fight and nap with the best of them - a power Superman surely would love to have.
