SA OC 30:2 rules that
One is prohibited to put Tefillin on at night since he may forget
[that he has them on] and sleep in them. But if he puts them on before
the sun sets and it happens to get dark, even if he wears them the
whole night, it is permitted.
The Rambam (Hilchot Tefillin end of 4:11) rules this is a transgression d'oraita.
A person ...
If I understand the משכנות יעקב [in סי' ע"ט] correctly, he writes that krias shema would depend on the time locals get up and go to sleep, and it is not dependent on the beginning of the day or the night. Therefore, he writes that places that have a short night, only the sleeping habits are taken into account. He then takes it a step further, and tries to ...