The Gemara states that "Layla Lav Zman Sisit"-night is not the time to fulfill the Misva of Sisit. The Rambam understands this at face value to mean that no garments are ever required in Sisit at night. However, the Rosh understands that the Gemara is referring to garments designated for wearing at night, like a pajama. Accordingly, if a person wore his regular "day" clothes at night, the Rosh would rule that he is obligated in Sisit, whereas the Rambam would hold that he is exempt. Conversely, if one would wear pajamas during the day, the Rosh would hold that he is exempt, whereas the Rambam would hold that he is obligated.
Although the Bet Yosef implies that he agrees with Rambam, in his Shulhan Aruch (18:1) he cites both opinions, without ruling like either of them. Hacham Ovadia understands that Maran was indecisive who to rule like, and therefore the Halacha must adopt the stringent outcome of both opinions. That is, if one wears a night garment during the day, it must have Sisit, in accordance with the Rambam. If one wears a day garment at night, it must have Sisit, in accordance with the Rosh. However, in both cases, a Beracha is not recited because of the principle "Safek Berachot L’Hakel"-when there is a doubt, a Beracha is not recited.
Therefore, if someone wants to put on a Tallit after sunset, he should not say a Beracha, because perhaps the Halacha is actually like the Rambam that even "daywear," like a Tallit, is exempt from Sisit at night. That is why on Ereb Yom Kippur, one should arrive in Shul before sunset so that he can recite a Beracha on his Tallit. Interestingly, Hacham Ovadia ruled that if one really wants to make a Beracha on his Tallit after sunset, he has what to rely on if he does so within thirteen and a half minutes after sunset.
If a person prayed Arbit early, before sunset, it is already considered nighttime for him, and he may no longer make a Beracha on a Tallit, even if the sun has not yet set. Doing so would create a "Tarte D’Satreh"-a internal contradiction: If it is already night to pray, how can he wear a Tallit? This can occur at a wedding in which the Hatan prayed Arbit early; under the Huppa, he may no longer recite the Beracha of "L’hitatef" on his new Tallit.
The Kaf Ha’haim (Rav Yaakob Haim Sofer, Baghdad-Israel, 1870-1939) and the Hida (Rav Haim Yosef David Azulai, 1724-1806) write, based on the Arizal, that one should not remain wearing his Tallit after nightfall. Therefore, after a fast day, one should remove his Tallit before Arbit. Although the Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) cites Poskim that one should remain in his Tallit for Arbit of Mosa’eh Yom Kippur because we are still likened to angels, Hacham Ovadia in Yalkut Yosef cites many Poskim who disagree and rule that the custom is to remove the Tallit before Arbit.
SUMMARY
One should not recite a Beracha on Sisit after sunset, or after praying Arbit before sunset.
One should not remain wearing his Tallit at night even on Mosa’eh Yom Kippur.
