If you actually feel like you might fall asleep, like maybe you're sitting in a meeting and they turn the lights off, or you're in a classroom and they turn the lights off to show a movie or something on the projector, you might say, I'm drowsy or I'm sleepy.
如果你真的覺得自己可能會睡著,比如你坐在會議上,他們把燈關了,或者你在教室裡,他們把燈關了,用投影儀放電影什麼的,你可能會說,我昏昏欲睡,或者我困了。