There’s a scene in the movie, The Family Man, where Nicolas Cage is sitting in a chair and trying to stay awake, because he knows that when he falls asleep his “glimpse” will be over.
The moments just before passing out are like that. Assuming that you’re not panicking, you’re vibrantly aware of everything around you — colors, smells, etc., because you don’t know if you’re just passing out or this is Game Over.
The end of a lucid dream can also be like that. You can be in the dream, know that you’re dreaming, and then know that you’re starting to wake up. You don’t want to leave, but you don’t have a choice, so you pay great attention to the environment because you know that you may never see it again.
To the best of my knowledge, all of those are also the correct mental state for Zen and mindfulness meditation. As Shunryu Suzuki says, “The true practice of meditation is to sit as if you are drinking water when you are thirsty.”