Complex sentence: On Richard's insistence, they go for a late-night walk and Natalie seems to cheer up, but on waking in the middle of the night, due to loud music from the neighboring apartment, Richard finds Natalie still awake and still extremely depressed.
Split into simpler sentences: On Richard's insistence, they go for a late-night walk and Natalie seems to cheer up. Richard stays over, and on waking in the middle of the night due to loud music from the neighbouring apartment, he finds Natalie still awake and still extremely depressed.