Having just changed the clocks from Daylight Savings Time to Standard Time, there is a temporary reprieve, as daytime now begins one hour earlier. However, the reprieve is only temporary, as daytime gets later rapidly over the course of the ensuing few weeks. In fact, after several weeks, the times are almost as late as they were before the clocks are changed.
 The Bi'ur Halacha (89:1 s.v. v'im hispaleil) points out that daybreak is not the time at which the "Morning Star" is visible; that star is visible considerably before daybreak.
 Bi'ur Halacha 89:1 s.v. yatza.
 Time for performing mitzvos.
 The mitzvah of talis is a daytime mitzvah (see Shulchan Aruch Orach Chaim 30:1) and, therefore, cannot be performed at night (S.A. O.C. 18:1). The mitzvah of tefilin, however, is technically a 24/6 mitzvah (Shabbos excluded), and Biblically may be worn at night. However, the Rabbis were concerned that if one were to wear tefilin at night, one might fall asleep while wearing them and flatulate, which is forbidden while wearing tefilin. Therefore, they forbade donning tefilin at night. However, since the only reason for not wearing them is the concern of falling sleep, one who woke up from his nighttime sleep may don them even before alos hashachar (S.A. O.C. 30:3; Mishnah Berurah 30:11). He may not, however, recite a b'rachah until misheyakir (ibid.; cf. Igros Moshe O.C. 1:10).
 S.A. O.C. 30:1. The zman for donning talis actually begins at the time at which it is light enough to recognize (i.e., distinguish) between the white strings and t'cheiles strings of one's talis (S.A O.C. 18:3). However, this time coincides precisely with the time at which it is light enough to recognize one's casual acquaintance from a distance of four amos (M.B. 18:9). (It should be noted that according to the Rema (O.C. 18:3), b'dieved, the zman for talis (but not tefilin) begins at alos hashachar. Thus, on Shabbos, when one dons only talis but not tefilin, according to the Rema one could begin davening with one's talis and recite the b'rachah on the talis after alos hashachar; see M.B. 58:10; Bi'ur Halacha58:1 s.v. zman K'rias Shema. L'chatchilah, it is most proper to not rely on this leniency of the Rema; M.B. 18:10.)
 A question arises: Since the zman is dependent on one's ability to discern and differentiate, perhaps on a cloudy day the zman should be later, and if there is moonlight or snow on the ground, which would reflect and magnify the light, the zman should be earlier. Indeed there are some Poskim - halachic authorities (see Eishel Avraham - Butchatch 58:1) who do take these factors into account, but most Poskim (Minchas Elazar, Nimukei Orach Chaim; Rav Chaim Kanievsky, cited in Da'as Noteh, page 347, #754; see also Sh"Ut Sho'eil Umeishiv 4:162) rule that the atmospheric and street conditions play no role in the zman; the zman is determined based on the degree of light had the day been clear.
 The mitzvos of Shema and Birchos K'rias Shema (the b'rachos of Shema) in the morning begin l'chatchilah at misheyakir (S.A. O.C. 58:1; M.B. 58:1). (It is preferable, however, not to recite them until shortly before haneitz hachamah). B'dieved, if one began reciting them shortly after alos hashachar but before misheyakir, one fulfills one's obligation (S.A. O.C. 58:3; Bi'ur Halacha 89:1 s.v. v'im hispaleil), but only if one does so infrequently (M.B. 58:19; 69:4), which is defined as only once a month. In cases of need (e.g., one needs to leave early for work), one may begin reciting them even l'chatchilah shortly after alos hashachar, even on a regular basis (ibid.).
 For example, in Eretz Yisrael, where it gets light quickly in the morning (just as it gets dark quickly in the evening), the time of misheyakir is closer to sunrise, while in New York, it is earlier.
 Igros Moshe O.C. 4:6.
 Eidus L'Yisrael (Rav Y. E. Henkin), Halacha section #4.
 The custom in Yerushalayim is to consider misheyakir as being either approximately at 52 minutes before haneitz hachamah, which corresponds to 11.5 degrees (Oros Chaim 1:5) or to 11.8 degrees (Otzros Yerushalayim vol. 81, p. 1289, side b). In Bnei Brak, many follow a zman of 40-45 minutes before haneitz hachamah, which corresponds to 10.2 degrees.
 These zmanim change according to the seasons. One can find these zmanim in two popular calendars, which are produced by: 1) Vaad L'chizuk - Rabbi Premock (718-851-1314); 2) MyZmanim (www.myzmanim.com).
DISCLAIMER: Not all details and aspects of the halachic issues discussed can be expressed fully in this limited format, and a small change in circumstances can change the halachic outcome. Accordingly, for one's personal situation, one is advised to ask a Rabbinic authority, and to not rely on the information presented herein.