There are a few details not really thought about in my series. Though, almost all major battles are at night. Why?
In the main series, the lead is strongest at night. The night is their domain, there are really just a few times you see a major battle in the day.
In Book 5, 6, 7.
Specifically Book 5, and 6. It became more like staple of the series to have the major battles at night, hence it acts as a reference
Book 2, when they awake is after they are killed and sent between the living and the dead for the start of their trials.(They cannot truly live nor die).
It is right when a purge is happening, and right before the weapon of the gods would come.