i think that it has to do with the freedom he gets from life when he can actually fall asleep. because of his insomnia, his brain is constantly working and thinking about things like time going by too fast, and all he wants is an escape. this is hinted in the "please take me away from here", he wants to be able to take a break from the reality and think about fantastic things like millions of fireflies dancing around him. "it's hard to say that i'd rather stay awake when i'm asleep", he wouldn't give up the time he actually gets to sleep, because in his dreams, nothing is as it seems. which to him, is a good thing. he is sad when the fireflies leave because it means he has to go back to the same, sensical life he's been living.
i think that it has to do with the freedom he gets from life when he can actually fall asleep. because of his insomnia, his brain is constantly working and thinking about things like time going by too fast, and all he wants is an escape. this is hinted in the "please take me away from here", he wants to be able to take a break from the reality and think about fantastic things like millions of fireflies dancing around him. "it's hard to say that i'd rather stay awake when i'm asleep", he wouldn't give up the time he actually gets to sleep, because in his dreams, nothing is as it seems. which to him, is a good thing. he is sad when the fireflies leave because it means he has to go back to the same, sensical life he's been living.