The Collaborative International Dictionary of English v.0.48:

Restful \Rest"ful\ (r?st"f?l), a.
   1. Being at rest; quiet. --Shak.
      [1913 Webster]

   2. Giving rest; freeing from toil, trouble, etc.
      [1913 Webster]

            Tired with all these, for restful death I cry.
                                                  --Shak.
      [1913 Webster] -- Rest"ful*ly, adv. -- Rest"ful*ness,
      n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

restfully
    adv 1: in a restful manner; "the streets are restfully sunny and
           still for the town is at mass" [syn: restfully,
           quietly]