The New York Times (August 17) reported that the N.F.L. has decided to cracked down on several players who wear fierce and funky face masks to make them look tougher or cooler under the headline, “Uniform police give face masks a closer look.”
The N.F.L. considers the use of such accessorized helmets without a legitimate medical reason violates the league’s uniform policy. http://www.nytimes.com/2013/08/18/sports/football/NFL-uniform-police-give-face-masks-a-closer-look.html?ref=sports?src=dayp&_r=0 

The article reads:

> “Gone is Brian Orakpo’s corrugated headgear, modeled after the Batman
> villain Bane, and Darnell Dockett’s square-bottomed mask, which looked
> oddly like a panini grill.  Tuck’s face mask fans, however, **can rest
> easily.** He provided N.F.L. officials with documentation, signed by the
> team doctor and head athletic trainer, certifying that his mask
> protected his neck by preventing opponents from grabbing it.”

I was drawn to the phrase, “Tuck’s face mask fans can rest easily.” 
I can find the idiom, “rest ease” in the meanings of “(1) not to worry - CED” and “(2) go to sleep without worries – OED” in any of English dictionaries at hand, but no dictionary carries “rest easily” as an idiom, because I think, it’s a simple combination of verb and adverb. 

However, I’m interested in what’s the difference is between “rest ease’ shown in “This insurance policy will let you rest easy “ shown as a sample in OED and “rest easily” in “Tuck’s face mask fans can rest easily” in the above quote.

 Does the addition of two letters - ‘ly’- make any difference? If it does, what’s the difference?