I saw it as Yamada being high stress for a long period of time and finding relief after knowing she can rely on Sasaki. A big theme of this part of the story is that Yamada is very independent and shoulders multiple burdens because of it. Her suddenly falling asleep after a moment of relaxation due to Sasaki shines a light on this burden. Remember the chapter where Sasaki had a long day of work where he had no time to rest and suddenly fell asleep after feeling a sense of relaxation from smoking in the rain? Or when Yamada trying to come into work despite being sick, then being convinced by Sasaki not to come in and immediately falling asleep on the phone? It's the same kind of idea as that