Being able to see trees while recovering from surgery – as through a hospital window – increases a patient's pain thresholds, requiring less pain relievers and shortening recovery time.
Ulrich, R. S. (1999). Effects of gardens on health outcomes: Theory and research. In C. Cooper-Marcus & M. Barnes (Eds.), Healing Gardens: Therapeutic Benefits and Design Recommendations. New York: John Wiley, pp. 27-86.