"Blessed are You haShem. our God, King of the universe, Who gives strength to those who are exhausted. "
Following the same idea of birkot hashaḥar, this blessing too describes what we do or experience when we wake up in the morning. In this case, the feeling that last night we were exhausted and, thanks God, in the morning we feel reenergized and ready to start a new day.
Our rabbis were very conscious of the positive effects of sleep for the renewal of our energy. Analyzing the verse "And God saw all He had made and, behold, it was very good" (Gen. 1:31) R. Shimon bar Yoḥai said: "very good" refers to sleep. As explained by R. Menashe ben Israel in The Conciliator: "Sleep is good for although it suspends the operation of the nobler faculties of the soul, yet the repose obtained by it gives fresh vigor and strength for study and the exercise of the arts and virtues."
There is a technical issue with this particular blessing: unlike all other blessings we recite today in birkot hashaḥar, this blessing was not mentioned in the Talmud or Maimonides. The bet yosef actually questions its recitation and in some Sephardic Siddurim this berakha is not found.
In his book Keter Shem Tob (p.19) , Rabbi Shem Tob Gaguin (see here) says that in his opinion this berakha must have been formulated "in times of religious persecution, when the Jews were mistreated and tortured to renounce to their faith and convert to a different religion." The Jews still persisted because God renewed their courage and gave them the strength to endure such suffering. An additional proof for this idea seems to be the context in which the words noten laya'ef koaḥ appear in the Scripture (Isaiah 40: 29 and 31): "HaShem gives power to the faint...they shall mount up with wings as eagles, they shall run [escaping from the enemy] and not be weary, they shall walk and not faint"
Le'iluy nishmat Tzibia bat Biniamyn, Mrs. Geyran Hakimi, z"l.