What do you do if you fall asleep in your tallis. When you wake up do you have to say a new bracha?
If one sleeps with a talis on during the day, no new berachah is made upon waking up.
Indeed, it is correct to sleep with one’s tzitzis on, to avoid questions of a berachah upon waking up, and also to continue performing the mitzvah of tzitzis throughout the day.
If one sleeps with a tallis katan on during the night, which the Magen Avraham (21:2) advises based on the Arizal (though this custom is not particularly common), one gets into a safek concerning making a blessing over the tzitzis in the morning (Mishnah Berurah 8:42).
This safek applies to bachurim, who do not wear a talis, and therefore make a berachah over their tallis katan. Having slept with the tallis katan, they cannot make a berachah over it, and it will also not help to take it off and to put it back on.
Rav Shlomo Zalman Auerbach zt”l (Halichos Shlomo p. 31, note 38) advises somebody (a bachur) who sleeps with a tallis katan to have two pairs of tzitzis, so that every morning he can change into the “other” one, and make a berachah.