All timezones are defined relative to GMT (usually in full/half hours), so all clock times in the US are, too.

Gotcha. Though in this day and age having our time based on the sun is a bit of an anachronism, as we work under artificial lighting and not in a field. And in any case Sunrise is not good enough, as some Nordic countries don't have any sunrise at all during winter.

What's Sunrise+1 in Iceland in December?

I'm sure for those extreme cases, their government would be able to determine the most suitable/healthy time to wake up for each day of year, using scientific methods.