Are Muslim required to observe full day of fasting where days (i.e. day light) are significantly longer than regular days in Arabia. I am referring to northern and southern hemisphere where days are very long in summer.
(Summarized from this source)
As long as day and night is distinguishable from each other, one is obligated to fast. Allah says (2:187 - Sahih International - Emphasis mine)
It has been made permissible for you the night preceding fasting to go to your wives [for sexual relations]. They are clothing for you and you are clothing for them. Allah knows that you used to deceive yourselves, so He accepted your repentance and forgave you. So now, have relations with them and seek that which Allah has decreed for you. And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]. Then complete the fast until the sunset. And do not have relations with them as long as you are staying for worship in the mosques. These are the limits [set by] Allah , so do not approach them. Thus does Allah make clear His ordinances to the people that they may become righteous.
However, if one is harmed because of the hardship of fasting, than it is permissible to break fasting and make up for it another day. Allah says (2-286 - Sahih International - Emphasis mine)
Allah does not charge a soul except [with that within] its capacity. It will have [the consequence of] what [good] it has gained, and it will bear [the consequence of] what [evil] it has earned. "Our Lord, do not impose blame upon us if we have forgotten or erred. Our Lord, and lay not upon us a burden like that which You laid upon those before us. Our Lord, and burden us not with that which we have no ability to bear. And pardon us; and forgive us; and have mercy upon us. You are our protector, so give us victory over the disbelieving people.