In the first chapter in Shulchan Aruch Admur discusses the detailed laws of Tikkun Chatzos. It is no longer accustomed amongst most Jewry to perform Tikkun Chatzos and hence the detailed laws written by Admur have been omitted from this chapter. Nevertheless parts of this Halacha have been brought for the reader to gain some general background of this tradition.
Awaking before midnight: [Although one may sleep until dawn, this is only the conduct of a Beinoni, however for] all G-d fearing Jews [which G-d touches their heart], it is proper to awaken by [midnight] and recite the Tikkun over the destruction of our temple and the exile of the Shechina, as during this period of time G-d also laments His destruction of the Temples [and for this reason ones prayer said at that time is accepted before G-d].
Having a rooster wake one up: It is proper to prepare a rooster to awaken one from his sleep by midnight emulating Rabbi Akiva who was accustomed to have a rooster even when traveling in order to wake him by midnight. If this is not possible then one should hire someone to wake him up.
The auspiciousness of this time: The auspiciousness of this time is elaborated on in the Zohar. As well the Talmud says that this is a time of appeasement above and that anyone who learns Torah at night the Shechinah resides opposite him, and a “string of kindness” is drawn upon him for that day. One who does so is also called a servant of G-d. The auspiciousness of the time of midnight is applicable at all times and in all places.
The order of Tikkun Chatzos:
One would awaken close to midnight and sit barefoot near the Mezuzah of the doorpost. He places ash on his forehead in the area of the head Tefillin and then recites the paragraph of “Lasum Laavleiy Tziyon”. He is then to stand and recite the verses of “Hisnaariy Meiafar Kumiy”. After the Tikkun, if it is still before midnight, one should study Mishnayos as whoever studies codified laws is granted a portion in the world to come. After midnight he is to study passages from the Zohar which discuss the prayers recited during the Tikkun.
The custom today of Tikkun Chatzos:
It is no longer accustomed amongst most Jewry to perform Tikkun Chatzos. The current Chabad custom is not to say it. Nevertheless one is to perform the service of Tikkun Chatzos during his recital of Kerias Shema Sheal Hamita. Likewise if one is awake near Chatzos he should not go to sleep and is to rather use the auspicious time to learn Chassidus, or learn a Sicha or perform an accounting of the soul.
 Basra 1/2-3; Kama 1/8-9; Siddur; See Kaf Hachaim 1/7-24; Piskeiy Teshuvos 1/10-12
Siddur Hashkamas Haboker: “It is known the greatness of awaking by midnight which is an auspicious time above. One who is not able to awaken every night should at least do so the nights that he is able and thus be considered a servant of G-d. On a night that one is unable to awaken by midnight he should at least try to awake some time [Eizeh Shaah] prior to dawn”
 Tikkun Chatzos used to be performed by the masses, including even the simple folk. [Reishis Chochma Shaar Hakedusha 7] As the generations progressed the amount of people who performed Tikkun Chatzos gradually decreased to the point that only those that are spiritually exceptional and righteous continued to perform it.
The justification for not performing Tikkun Chatzos: The Ashel Avraham [Butchach 1/2] writes that if waking up for Tikkun Chatzos will affect ones health [sleep schedule] then he is not to do so even if there is the slightest doubt that it may affect his ability to function and health. He thus justifies the practice of many that do no wake up for Tikkun Chatzos. The Nimukeiy Orach Chaim [1/1] takes the Ashel Avraham to task for giving such a wide spread allowance to anyone with the slightest doubt of a health hazard as in this path one will avoid fulfilling all Torah and Mitzvos that may somehow possibly affect his health. The Nimukeiy Orach Chaim hence suggests that if one is unable to awaken by Chatzos then he should stay awake until Chatzos and then perform the Tikkun.
The Chabad custom: In the Siddur, Admur writes that every person which the fear of heaven touches his heart is to remove the mask of shame and perform Tikkun Chatzos in public gatherings of ten people or less or even alone. This was practiced by many Chassidim in the times of Admur to perform Tikkun Chatzos with a Minyan in Shul. [Igros Kodesh Rayatz 8 p. 341] With the spread of Chassidism in the times of the Alter Rebbe the custom then spread to recite Tikkun Chatzos alone. [Likkutei Dibburim 3 p. 757] The custom then became not to do so every day. [Kuntrus Hatefilah 11 p. 24] The custom then became not to do so at all. [Sichas Kodesh 5711 p. 451 in answer to Rav Yitzchak Hutner; Rebbe Rashab in Toras Shalom p. 6 “today it is thought that Tikkun Chatzos is only to be performed by great Chassidim”] The Rebbe there stated that the fulfillment of Tikkun Chatzos is now done through learning Chassidus after Chatzos. The Rebbe once stated in Yechidus that one does not have to sleep before Chatzos and hence if he already recited Kerias Shema Sheal Hamita and it is near Chatzos, he should use his time to learn a Sicha or do a Cheshbon Nefesh. [See Toras Menachem 3 p. 357; Nimukeiy Orach Chaim ibid] To note however that the noted Chabad Posek Rav A”C Naah records in his Sefer Ketzos Hashulchan 1/2-3 the laws of Tikkun Chatzos and does not make mention of any change from the custom.
A Tradition from the Baal Shem Tov: It is said in the name of the Baal Shem Tov that he nullified the performance of Tikkun Chatzos from the common folk and instituted it be done only by distinguished individuals that truly lament the exile of the Shechina. In the writings found in “Geniza Charsanit” it is brought that the Baal Shem Tov already purified the first half of the night.
 Basra 1/2
 Basra 1/2
 Basra 1/ 2, and Siddur.
Ruling of Admur in Kama: In the Mahadurah Kama the time of “midnight” is not mentioned, rather Admur says that it is proper for one to awaken before daybreak, and it is best to awaken at the end of any of the three Mishmaros to say the Tikkun, as at that time G-d remembers the destruction. [1/8] Thus there is an argument between the Kama and Basra as to when G-d remembers the destruction. At the times of the Shulchan Aruch they still did not know of the greatness and auspiciousness of the time of midnight which is based on mainly Kabalistic sources, and thus in the 1st edition of the Shulchan Aruch of Admur he does not make mention of midnight. However in the second edition which follows more the Kabalistic rulings it is mentioned that midnight is the proper time of awakening. [See Machatzis Hashekel 1/4; Piskeiy Teshuvos 581/2 regarding the time of Selichos].
 Kama 1/8 regarding the time of the end of each Ashmura.
 Basra 1/3
 Basra 1/3
 Basra 1/8
 Basra 1/2
 Kama 1/8 based on Shlah brought in M”A
 To note however from Kama 1/8-9: “One should learn the Oral Torah at this time.” And “One who knows how to learn should not elongate the supplications, and instead should spend his time learning Torah, as learning Torah is greater than Iyun Tefilah.”
 See Hiskashrus 256 and 601
 Sichas Kodesh 5711 p. 451 in answer to Rav Yitzchak Hutner; Rebbe Rashab in Toras Shalom p. 6 “today it is thought that Tikkun Chatzos is only to be performed by great Chassidim”.
 So is accepted amongst Chassidim. See Hiskashrus ibid
 Sichas Kodesh 5711 p. 451 in answer to Rav Yitzchak Hutner
 Toras Menachem 3 p. 357 brought in Hiskashrus 601