On fast days that begin in the morning, (Asara B'Teves, Taanis Esther, Shiva Asar B'Tamuz, Tzom Gedalya) if you intend to eat when you awake, until what time may you eat in the morning? Does it depend on Shaos Zemanios?
Re the first response, the halacha as described in Shulchan Aruch 564 is quite different than presented. First, fasts always start at amud hashachar (72 minutes l'chumra, 90 minutes super l'chumra). The only exception is where one fell into a deep sleep with no intention of eating afterward. However, most people either set their clock to davka wake up early, or else hope that they'll wake up early enough to have a bite before the fast starts. In that very common case (possibly 100% nowadays), everyone agrees one may eat. Moreover, the Rema there on 564 says that we always have an intention to drink during the night, so there's no need to make a t'nai (condition) in order to drink if one wakes up unexpectedly early. So even for the machmirim, one can drink right before amud hashachar.
The night ends at "Alot haShachar", the "rising of the morning". It has two halachic definitions: Most rule it is 72 minutes before dawn, some use the solar equivalent--16.1o (degrees) below the horizon. The latter would come dawn+72 min if the sun were up for exactly 12 hours that day. In the summer it would be longer, in the winter, later. Others use 90 solar minutes.(I'm not sure about this source)