The Halo Ring in CE emits an aura, such that it makes creatures fall asleep.
Based in purely the soundscape it could be concluded that the sounds and environment are sleep-inducing. i.e. the doors sigh as they open, and the weather is gentle. That's why on some parts of assault on the control room, the grunts fall asleep if you kill all senior enemies (jackals, Elites etc.) and leave the vicinity for a long time.
As in regards to grunts asleep in the presence of elites, maybe grunts have to sleep sometimes such that when you appear they are as such.
Additionally Chief infiltrates the control room area from underground, the Covenant don't expect this, they expect assault from above (probably Pelicans). If your're in a bunker, you'd be sleeping.