﴿ إِذْ نَادَىٰ رَبَّهُۥ نِدَآءً خَفِيًّۭا ﴾

“When he called out to his Sustainer in the secrecy of his heart,”

when idh is semantically connected to rahma ‘mercy’ he called out to his Lord a call entailing a supplication in secret in the middle of the night because this time of the night invites a faster response to one’s supplication.