The time of kriat shma is learned basically from the pasuk "בשכבך ובקומך", meaning the time people sleep and the time people wake up.

2000 years ago people acted according to the daylight. Nowadays we have electricity. The time should be adjusted to the normal working hours. Let's say in the winter, people don't start to sleep at 5 pm, nor start to wake up at 7:00 am.

The opposite question is in the summer. If i am living in the UK or canada, why i am permitted just to say shma at 10 pm, people start to go sleep earlier. Or why can i say shma already at 4 am ?

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    Part of the answer might be that certain things you are asking are not derived from the words you quote but from the idea of "evening and morning" which are dependent on the sun rise/set cycle and not the sleep cycle. If everything were based just on rising and lying down, then could someone who works overnights claim that the sabbath should start on a different schedule?
    – rosends
    Feb 10, 2016 at 11:51
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    The source is a derasha. If we had the Sanhedrin, we would probably not have made that derasha. Upon reestablishment of the Sanhedrin, we can change the derashao keep it up with the times. Until then, we are mostly stuck. (BTW IIRC some hold that the time is only rabbinic; accordingly the verse is a mere asmachta, and the time was never actually dependnt on it. If so, qushya meiqara leita.)
    – mevaqesh
    Feb 10, 2016 at 14:49
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    @DavidMichaelGang in relation to human davening or just as a measure of a part of the day? If, as per judaism.stackexchange.com/questions/30485/hashem-studies-torah the first three hours are when hashem is recreating the world, prayer might be appropriate for that window.
    – rosends
    Feb 10, 2016 at 15:09
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    The boundaries for normative behavior haven't changed - we don't look at the average waking individual, but on what melachim do - people with no external requirements upon them ("masters of their own time," if you will). The closest analog in the modern era are probably retired individuals, who tend to sleep till mid morning and retire early evening. But take this with a krovetz of melach Feb 10, 2016 at 15:58
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    In Shu"t Mishkenos Yaakov #79 he brings up that issue. See hebrewbooks.org/pdfpager.aspx?req=1095&pgnum=84 Feb 10, 2016 at 20:27


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