The religious day as evidenced by Avestan Yasna literature starts at hávan, SUNRISE or time to extract/press the nectar of “haómá” plant.
Yet, if we look at the poetic gathas and khorde avesta prayers for the daily prayers we realize the following:
In the enchanting gathas fourth line of Yasna 44.5, there is mention of 3 divisions namely: i) úsháw Dawn ii) arémpithwá High Noon iii) and khshapá Night or time to sleep/repose.
úsha.hin” is related to sanskrit ushah/ushas meaning Dawn. Old.Norse. austr “from the east, the direction in which dawn breaks. ” Proto Indo European aus- “dawn.” Old.Irish. usah, Lithuanian. auszra “dawn,” lit. “to shine.”
úsha.hin therefore would have been dawn and pre-dawn prayers, completed by SUNRISE with addition of yet another Gāh specifically for celebration of sunrise. The daily prayers appertaining to úsha.hin Gāh; praise the glory and beauty of DAWN that spreads over 7 dominion/kheshvar of the earth; intelligence, understanding and brilliance; the light of the house/dwelling, and the brilliance/glory of Ahurmazd with 5 out 7 of the most prominent Ameshá speñtá. Then, during hávan Gāh the praise is completed by naming the remaining 2 of the 7 MOST PROMINENT Ameshá speñtá.
The hávan prayer then names/praises Yasna 54.1 or “airyemá ishyö” or conclusion formula of the poetic gathas; then goes to name/sing the praise of the “haptanyg-háiti” or the seven chapters (Yasna 35-Yasna 42.)
The key about hávan prayer is that on stanza 5, we salute ahüric questioning, adventure, quest and discovery. That is a clear indication that religious day starts with hávan or SUNRISE when the sun//light is out completely.
The separation of the aforementioned Gāhs seem to have stressed the importance of late night/pre-dawn prayers.
Also, Uzayäirin and Aiwi-srüthrim prayers could be said shortly after each other, thereby reducing prayer/meditation times to 3 major sessions of fivefold prayers.