Gemara

Rabah bar Rav Huna: A Kal va'Chomer teaches that someone wearing Tefilin must constantly touch them! Even though the Tzitz has Hash-m's name on it only once, the Kohen Gadol may not have Hesech ha'Da'as (lapse of awareness) while wearing it - "V'Hayah Al Mitzcho Tamid";

Hash-m's name is written many times in Tefilin, all the more so Hesech ha'Da'as is forbidden!

Question: The Tzitz is more Kodesh in some ways (Hash-m's name is visible when the Tzitz is worn, and it is only worn by the Kohen Gadol in the Mikdash; these do not apply to Tefilin) - this refutes the Kal va'Chomer!

Answer #1 (Tosfos u'Mah): It is not a proper Kal va'Chomer. Rather, we merely show why it is fitting for Chachamim to enact this.

Answer #2 (Ritva): The Isur of Hesech ha'Da'as depends only on Hash-m's name.

Sukah 26b: One may sleep Arai (haphazardly) while wearing Tefilin.

Abaye: R. Yosi forbids young men to sleep with Tefilin when their wives are there, lest they come to Hergal Davar (relations, while wearing them).

Sukah 46a: Rabanan of Rav Ashi's academy would bless on Tefilin whenever they touched them.

Rishonim

The Rif and the Rosh (Hilchos Tefilin printed after Menachos, Daf 9a and Siman 18, respectively) cite Rabah bar Rav Huna verbatim.

Rambam (Hilchos Tefilin 4:13,14): Someone in pain or whose mind is unsettled may not wear Tefilin, for one may not have Hesech ha'Da'as. One must constantly touch his Tefilin while wearing them so that he will not cease to think about them even for a moment. Hash-m's name is written 21 times in hand Tefilin, and the same in head Tefilin, therefore they are more Kodesh than the Tzitz.

Question: Why didn't the Poskim bring the practice of Rabanan of Bei Rav Ashi to bless on Tefilin whenever touching them?

Answer #1 (Ran Sukah 22b): There is a fixed time interval after which one must touch Tefilin. One may not bless for touching after a shorter interval. They knew the Shi'ur but we do not.

Answer #2 (Ran, ibid.): They would bless when returning them to their place if they slipped out of place.

Question: Sukah 26b permits wearing Tefilin while sleeping - he is not aware of them at the time!

Answer #1 (R. Yonah (Berachos 14b), brought in Rosh (Berachos 3:28)): The Hesech ha'Da'as forbidden while wearing Tefilin is frivolity. Surely, one may wear Tefilin while engaging in work in awe of Hash-m, even though he ceases to think about Tefilin, for it is a Mitzvah to wear Tefilin all day!

Answer #2 (Sha'agas Arye 40): The Ran suggests that one must touch Tefilin at regular intervals. Perhaps Hesech ha'Da'as is forbidden only for a longer Shi'ur, and this Shi'ur is more than the length of time of Arai sleep permitted (the time to walk 100 Amos, this is about half a minute). Even though we learn from "V'Hayah Al Metzach Aharon Tamid", we find that 'Tamid' does not always mean every moment, e.g. regarding the Korban Tamid (it is only during the day) or the Menorah (it is only at night).

Question: Why was fixed sleep forbidden lest one pass gas - it should be forbidden lest one have Hesech ha'Da'as for the Shi'ur!

Answer: Hesech ha'Da'as is forbidden only mid'Rabanan (according to Tosfos above), whereas gas is forbidden mid'Oraisa (it is a disgrace to Tefilin).

Tur (YD 388, citing Ramban): A mourner (after the first day) should settle his mind so he may wear Tefilin without Hesech ha'Da'as; he should not wear Tefilin at the time of crying and eulogy.

Poskim

Shulchan Aruch (OC 28:1): One must constantly touch his Tefilin while wearing them so that he will not have Hesech ha'Da'as.

Question: The Kal va'Chomer does not teach about touching them! Also, it is unreasonable that one must keep his hand on his Tefilin the entire time he wears them!

Answer (Bach DH va'Yemashmesh): We learn that one must touch his Tefilin whenever he remembers them in order that he will not cease to think about them.

Magen Avraham (25:21, citing SHLa"H): Since one may not cease to think about them, if they slipped from their place during prayer he need not bless again when returning them, for it is as if he removed them with intent to return them.

Tur (OC 44): Only frivolity is considered Hesech ha'Da'as.

Magen Avraham (44:2): According to this, one need not touch them constantly during prayer, for in any case frivolity is forbidden at the time.

RaM me'Rotenburg (cited in Beis Yosef 38 DH v'Chosav): One who cannot refrain from passing gas may not wear Tefilin; all the more so, one who cannot refrain from lustful thoughts may not wear Tefilin!

Rebuttal (Beis Yosef, ibid.): The Gemara (Sukah 26b) forbids youths lest they come to Hergal Davar, i.e. relations - but mere thoughts are not a reason to forbid!

Defense #1 (of RaM me'Rotenburg - Beis Yosef): Perhaps the RaM me'Rotenburg explains that Hergal Davar refers to lustful thoughts. The Yerushalmi says that Tefilin require a body clean of evil thoughts.

Defense #2 (Magen Avraham 38:4): Perhaps even though it is forbidden to wear Tefilin when one has evil thoughts, we would not forbid wearing Tefilin if it is doubtful whether or not he will have such thoughts.

Beis Yosef (ibid.): Rather than exempt people from Tefilin due to evil thoughts, they should be trained to veer from such thoughts and accept Ol Malchus Shomayim.

Question #1 (Sha'agas Arye 39): If only frivolity is Hesech ha'Da'as, why does the Tur himself (YD 388) forbid a mourner?

Question #2: Why must one constantly touch Tefilin if only frivolity is forbidden?

Answer (Sha'agas Arye, ibid.): One must constantly touch Tefilin so he will not forget that he is wearing them and come to frivolity.

Question #3: Our source to forbid Hesech ha'Da'as is from the Tzitz, which must be "Al Mitzcho Tamid", i.e. always on his mind! Some say that the Tzitz may be worn only at the time of Avodah - surely frivolity is forbidden then, and always in the Beis ha'Mikdash. Even if it is permitted to wear the Tzitz outside the Mikdash, we cannot say that the Kal va'Chomer teaches about this. Tosfos proves that the Kal va'Chomer to Tefilin is only mid'Rabanan - even without the Kal va'Chomer frivolity is forbidden mid'Rabanan, for it removes Yir'as Shomayim!

Kaf ha'Chayim (28:2): Bach agrees with R. Yonah that only frivolity is Hesech ha'Da'as; nevertheless, the ideal Mitzvah is to constantly think about Tefilin. The Ari Zal allows to cease thinking about Tefilin when learning Torah or praying Shemoneh Esre, but forbids at all other times, even for a moment, even during other prayers. It is good to have a reminder to think about Tefilin next to one's Sidur or on every page of the Sidur.

Kaf ha'Chayim (28:3): Since we do not wear Tefilin all day, we should be stringent not to have any Hesech ha'Da'as for the short time we wear them.