Gemara

(Tana d'Vei R. Chiya): The Shechinah faces anyone who engages in Torah at night -- "Kumi Roni ba'Lailah... Nochach Pnei Hash-m."

Bava Basra 121b (Beraisa - R. Eliezer ha'Gadol): From the 15th of Av and onwards, (the nights get longer, so) one who increases learning at night will live longer. One who does not do so, Yasif.

(Rav Yosef): Yasif means that his mother will bury him.

Eruvin 65a (Rav Yehudah): Night was made for sleeping.

(Reish Lakish): The moon (i.e. its light; most explain, night) was created only for Torah.

People told R. Zeira that his teachings are very clear.

R. Zeira: I learned them during the day.

(Rav Nachman bar Yitzchak): We are like day laborers.

Sanhedrin 92a (R. Elazar): If Torah is not heard in a house at night, it will be burned - "...Te'achlehu Esh Lo Nupach Yera Sarid b'Ohalo."

Avodah Zarah 3b (Reish Lakish): If one engages in Torah at night, Hash-m bestows grace on him during the day - "Yomam Yetzaveh Hash-m Chasdo uva'Laylah Shiro Imi."

Menachos 110a - Question: What is the meaning of "Avdei Hash-m ha'Omdim b'Veis Hash-m ba'Laylos"? (Avodah is primarily during the day!)

Answer (R. Yochanan): Chachamim engage in Torah at night. It is considered as if they engage in Avodah.

Berachos 4b (Beraisa): One should not come home from the field and say "I will eat and sleep a bit, and later I will say Shma and pray." Perhaps he will sleep the entire night!

Rather, when he comes from the field, he goes to the Beis ha'Keneses. He reads (Tanach) or learns Mishnayos, whatever he usually learns. (In the proper time) he reads Shma and prays.

Rishonim

Rambam (Hilchos Talmud Torah 3:13): Even though it is a Mitzvah to learn by day and at night, most of one's learning is only at night. Therefore, one who wants to merit the crown of Torah will be careful with all his nights and not lose even one in eating, drinking and conversation.

Tosfos (65a DH Ela): Night was made for sleeping, e.g. the (short) summer nights.

Me'iri (65a DH Kol): Night was made for sleeping, to enable people to learn during the day without sleeping. One's primary learning is during the day. This is why the Gemara says 'we are day laborers.' This is if one finds that sleep overtakes him. If he is wakeful, he should fix his nights for Torah. About such a person it says that night was made for Torah.

Poskim

Shulchan Aruch (OC 238:1): One must be careful about learning at night more than during the day. There is great punishment for one who neglects it.

Bach (1): The Acharonim have a tradition 'from Shichvi until Kumi, Shichvi, and 'from Kumi until Shichvi, Kumi.' I.e. from Shevu'os, when we read Megilas Rus in which it says "Shichvi Ad ha'Boker", until Tish'ah b'Av, when we read Eichah in which it says "Kumi Roni va'Laylah", Shichvi (lie), i.e. one need not rise before dawn in order to learn, for the nights are short. From Kumi until Shichvi, i.e. from Tish'ah b'Av until Shevu'os, Kumi, i.e. one must rise before dawn in order to learn, since the nights are long.

Magen Avraham (Reish ha'Siman): The Levush says that one must rise before dawn in order to learn from Av 15 until Iyar 15. Some say that one should sleep for eight hours. I say that it depends on the person.

Prishah (1) and Shach (YD 246:25): Even in the short summer nights, which are primarily for sleeping, one must learn a little, just like in winter, when the nights are primarily for learning, one must also sleep [a little].

Birkei Yosef (2): Gurei ha'Ari Zal says that one should not learn Mikra (Tanach) at night. This is clear from the Medrash (Yalkut Sof Ki Sisa) that says that when Hash-m taught Mikra to Moshe, Moshe knew that it is day, and when Hash-m taught Mishnah, he knew that it is night. This proves only that Moshe did not learn Mikra at night, but perhaps just like we learn also Mishnah and Gemara during the day, we may learn Mikra at night! The Targum of Kumi Roni ba'Lailah is 'engage in Mishnah at night.'

Mishbetzos Zahav (1): It is proper to divide one's learning (equally) among Mikra (Tanach), Mishnah and Gemara at night, just like during the day.

Sha'ar ha'Tziyon (1): Be'er Heitev brings that one should not learn Mikra at night. The Mishbetzos Zahav connotes that one may. Even the stringent opinion holds that there is no Isur, just l'Chatchilah it is better to learn Mikra during the day.

She'elas Rav (of ha'Gaon R. C. Kanievsky Shlita, 2:25:16): It is better to say Shnayim Mikra v'Echad Targum during the day.

Ashrei ha'Ish (2:11:7, citing ha'Gaon R. Y.S. Elyashiv Shlita): One may say Shnayim Mikra v'Echad Targum at night.

Birkei Yosef (3): R. Yakov Tzemach says that one must engage in Torah for three hours after Ma'ariv before going to sleep.

Kaf ha'Chayim (1): The Zohar says that one who engages in Torah at night will not be afraid above or below or from bad occurrences, for he clings to the tree of life.

Kaf ha'Chayim (2): He must say the words, and not just learn in his heart, like the Zohar says.

Kaf ha'Chayim (3): The learning should primarily be after midnight, like the Zohar says.

Kaf ha'Chayim (4): One should learn also before midnight, in order to go to sleep amidst Divrei Torah. The Magid told the Beis Yosef to learn 10 chapters of Mishnayos before going to sleep, and then his Neshamah will be elevated (during sleep).

Kaf ha'Chayim (5): If one must go to sleep again after Tikun Chatzos (lamentations over the Churban, to be said at midnight), he should rise before dawn to join day and night through Torah.

Mishnah Berurah (1:9): After Tikun Chatzos, the first thing to learn is Mishnah. If one merits to learn Kabalah, this is a great time for it. The Acharonim have a tradition 'from Shichvi until Kumi, i.e. from Shevu'os until Tish'ah b'Av, one need not rise (for Tikun Chatzos and learning afterwards - PF.) Some say from Av 15 until Iyar 15. The Mekubalim say that one should always rise. One who is awake at night must be very careful to avoid secular talk. If one cannot rise at night due to his weak nature, or because if he rises at the end of the night he will sleep during the Tefilah, it is better that he sleep as much as he needs. In any case, he must be careful to rise an hour, or at least half an hour before the attendant calls people to come to the Beis ha'Keneses, so he will be able to proper himself to pray with the Tzibur and use the bathroom, if necessary.

Shulchan Aruch (YD 246:23): One who wants to merit the crown of Torah will be careful with all his nights and not lose even one in eating, drinking, conversation and similar matters, rather, only in words of Chachmah and Talmud Torah.

Rema: This is because most of one's learning is only at night.

Source (Beis Yosef (DH u'Mah she'Chasav Af): We learn from Medrash Rabah (5:7) on the verse "Kevutzosav Taltalim" (Shir ha'Shirim 5:11).

Gra (73, Likut): In the Medrash, R. Yochanan learns from "va'Takam b'Od Laylah" that Rinah (chanting) of Torah is only at night. Reish Lakish learned from "v'Hagisa Bo Yomam va'Laylah" that it is also during the day. He retracted and agreed with R. Yochanan.

Matanos Kehunah (on the Medrash): Some texts say Granah of Torah is only at night, i.e. the gathering and preservation.

Etz Yosef (on the Medrash): The Mitzvah to fix times for Torah is equally during the day, but it is primarily at night, when one is free to delve deeply into it. Reish Lakish said that when he would investigate Torah during the day, this would enlighten his eyes at night.

Rema: One should begin to learn from at night from Av 15 and onwards. One who does not increase, Yasif.

Shach (26): Yasif means that he will die prematurely.

Shulchan Aruch (24): If Torah is not heard in a house at night, it will be burned.

Bi'ur Halachah (155:1 DH Es): One must fix times to learn during the day and at night (YD 246:1). Therefore, it is proper that in addition to a fixed quota of learning after Shacharis, one should fix to learn also between Minchah and Ma'ariv. Through this he fulfills the Chiyuv to learn at night. Berachos 4b supports this.