After experiencing your first and second periods of deep sleep, your body enters wakeful sleep stages, like REM sleep or light sleep, which makes you vulnerable to common sleep disruptions.
July 5. The clouds of noon on the high Sierra seem yet more marvelously, indescribably beautiful from day to day as one becomes more wakeful to see them.
The captain concluded that Eustacia, feeling wakeful, had gone for a book, and would have dismissed the matter as unimportant if he had not also heard her distinctly weeping as she passed.