Repeating Shema at Night

1. The time to say Kriyat Shema at night is Tzet HaKochavim (3 small stars across the sky or 72 minutes after Shekiah). If the minyan was Davening earlier one can daven with them that’s if it’s after Shekiah (or it’s after Plag Hamincha and one already said Mincha before Plag Mincha) and then repeat Shema after Tzet HaKochavim without the Brachot.
2. When repeating Shema, it’s sufficient to say the first 2 paragraphs but a Yireh Shamayim should say all 3.
3. One should not rely on the Kriyat Shema Al HaMitah for several reasons including a) the Ashkenazic minhag is not to say all 3 paragraphs and b) a person doesn’t have kavana to fulfill Shema and also to accept the yoke of heaven when saying it. If one is insistent on using Kriyat Shema Al HaMitah as the repetition one should be careful to say all 3 paragraphs with kavana.
4. It’s proper to say Kriyat Shema within the first half hour after Tzet HaKochavim because of Zerizin Makdimin LeMitzvot (alacrity for מצות). If one didn’t say it then, make sure to complete it by Chatzot (midnight) and after the fact, one can still say it until Olot HaShachar. If one was in extenuating circumstances one may say it until HaNetz HaChama.

Sources

( V | T )	Specific parts of Prayer
Morning prayers	Birchot HaShachar - Birchot HaTorah - Korbanot - Kaddish - Pesukei DeZimrah - Barchu - Birchot Kriyat Shema - Kriyat Shema Amidah: Shmoneh Esrei - Mashiv HaRuach - Atta Chonen - Atta Chonantanu - Hashivenu - Slach Lanu - Refaenu - Barech Aleinu - Yaaleh VeYavo - Al Hanissim - Sim Shalom - 3 Steps - Chazarat HaShatz - Kedusha - Birkat Cohanim - Havinenu Post-Amidah: Kriyat HaTorah - Hagbah and Gelila - Tachanun, Ashrei, Aleinu, Shir Shel Yom
Other daily prayers	Mincha - Mariv/Arvit - Repeating Shema at Night - Bedtime Shema - Tikkun Chatzot
Additional prayers	Tefillat HaDerech - Mussaf - Hallel of Rosh Chodesh