The phrase "You'll be the last thing I think of before I fall asleep" suggests that the person occupies the speaker's mind as they prepare to sleep, highlighting their importance in the speaker's bedtime reflections.
Conversely, "and the first thing I think of when I wake up" underscores the person's role in the speaker's morning thoughts, signifying that they are the initial and foremost thought upon waking.
Overall, this statement beautifully conveys the idea of a continuous and enduring connection, where the person is consistently on the speaker's mind, both at night and in the morning. It emphasizes the depth of their affection and the significance of the person in the speaker's daily life.