one of my favorite scenes in ROTG was when Jack watched Sandy's dream sand and the line "Right on time, Sandman." Here we have Jack, usually so fun loving and cheerful, feeling depressed and confused. it broke my heart to see him so down, I just wanted to hug him! but then he was adorably cheered up by the dream sand and that spirit of fun and joy came right back! The happiness the sand brought back to Jack was "right on time." 
and I looked up what dolphins symbolize in dreams and got this description that fit Jack perfectly! I know the DW writers did this on purpose (which makes it even better!). if and when Jack sleeps, he dreams of dolphins - Head Canon accepted!
"To see a dolphin in your dream symbolizes spiritual guidance, intellect, mental attributes and emotional trust. The dream is usually an inspirational one, encouraging you to utilize your mind to its capacity and move upward in life. Alternatively, it suggests that a line of communication has been established between the conscious and subconscious aspects of yourself. Dolphins represent your willingness and ability to explore and navigate through your emotions."
btw, I'm sorry (but not really!) for only drawing ROTG fan art lately! ha ha! I'll get back to my other fandoms soon! (...maybe)
and I looked up what dolphins symbolize in dreams and got this description that fit Jack perfectly! I know the DW writers did this on purpose (which makes it even better!). if and when Jack sleeps, he dreams of dolphins - Head Canon accepted!
"To see a dolphin in your dream symbolizes spiritual guidance, intellect, mental attributes and emotional trust. The dream is usually an inspirational one, encouraging you to utilize your mind to its capacity and move upward in life. Alternatively, it suggests that a line of communication has been established between the conscious and subconscious aspects of yourself. Dolphins represent your willingness and ability to explore and navigate through your emotions."
btw, I'm sorry (but not really!) for only drawing ROTG fan art lately! ha ha! I'll get back to my other fandoms soon! (...maybe)
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And now that I think of it though, maybe your theory involving dolphins in dreamsand are part of the movie itself...
i think it is. you could say that Jack recovering his memories and finding his center, who he truly is, is establishing a line of communication between the conscious and subconscious. I'm pretty sure that whenever you touch dreamsand, even if you're not asleep, it shifts into what you dream of or a physical representation of your subconscious.
even if you look at that scene where the Guardians accidently get knocked out with dreamsand and start dreaming, it's symbolic. sure, it seems obvious that Toothfairy would dream of teeth, but according to the site I looked at, dreaming of glistening teeth signifies happiness and fulfilled wishes. and this was right after Tooth realized how happy she was being back in the field after 400+ years! and Bunny dreaming of carrots signifies abundance and fertility. and according to the behind the scenes and art book of ROTG, Bunny has the ability to make things come to life, hence he's the bringer of spring time. their dreams may have seemed obvious based on who they are, but in reality, they had deeper meaning! The Dreamworks people did their research!
thanks for the comment