To be more specific, there are 4 time zones in the contiguous 48 states: Pacific, Mountain, Central, and Eastern. Alaska is also one hour behind Pacific, and Hawaii (usually) one hour behind Alaskan. But Hawaii also doesn’t observe daylight savings time.
This also doesn’t account for how some states (like Arizona) don’t observe DST while Navajo Nation (in Arizona) does, while Hopi land (surrounded by Navajo) doesn’t.
Time is confusing.
The study isn’t from Oxford. It’s from a team of Chinese scientists (likely in China) who used a large dataset collected in the UK.
The study is published in The Journal of Clinical Endocrinology & Metabolism, which the Oxford Academic collects and reproduces for their academic press.