«فَسَرَتْ حُمَيَّا الْمَسَرَّةِ فِيهِمْ»
A heady wave of delight spread through them.
Sarat suggests moving or permeating; ḥumayyā an intense, heady state; masarra joy. Delight is presented as something travelling through the company.
The same people who had been drifting toward sleep now experience collective excitement. Personified joy makes the change vivid and almost physical.