As we have learned, the original institution of the Chachamim was to bless and give thanks for each and every act immediately upon deriving pleasure from it. In other words, as a person wakes from his sleep, he thanks Hashem for the soul He placed within him and says Elokai Neshamah; as he opens his eyes, he recites Poke’ach ivrim; as he stretches his limbs, he recites, Matir assurim; and so on with all the blessings in that manner. However, today the custom has changed and usually all Birkot HaShachar are recited consecutively.
One may wonder why the custom changed, for it is obviously more appropriate to thank Hashem immediately when the pleasure is received. In that way the process of arising from one’s sleep attains profound significance, as the blessings of thanks to Hashem accompany each and every stage of waking. Indeed, the Rambam rules that all Birkot HaShachar must be recited exactly as mentioned in the Talmud, each berachah in its appropriate time. There are some Yemenites who are accustomed to following his ruling even today.
However, the widespread custom is to recite all Birkot HaShachar at once, either in synagogue, or at home after relieving oneself and getting dressed. There are a few reasons for this. First, the Chachamim established that the chazan recites Birkot HaShachar out loud in synagogue so that the uneducated who do not know them by heart may fulfill their obligation as well. Also, even those who do know the blessings by heart may forget a blessing in the disorientation of waking, while if they recite them from a siddur in synagogue, they will remember to say them all. Further, we want to enhance the mitzvah and recite Birkot HaShachar in the most respectful way, with clean hands and while properly dressed, and therefore we delay the recital of the berachot until after all the preparations for prayer are finished (Shulchan Aruch 46:2). Moreover, there are people who find it very difficult to concentrate immediately upon waking up, and only after they dress and wash their faces are they able to recite Birkot HaShachar with kavanah (Seder HaYom).