For me it's a strange experience: I notice it almost only when ready to sleep, by day even if I focus to check if I can hear it I don't. And when I'm hearing it

For me it's a strange experience: I notice it almost only when ready to sleep, by day even if I focus to check if I can hear it I don't. And when I'm hearing it at about bedtime, I start yawning continuously and very "strongly" because after some tries, it disappears. Do someone has an explanation to this?