Your body has a lot of processes that run on a 24-24.5 hour clock, such as metabolism, melatonin and temperature, which are mostly synchronized and coordinated

Your body has a lot of processes that run on a 24-24.5 hour clock, such as metabolism, melatonin and temperature, which are mostly synchronized and coordinated by bright light exposure. You're designed to sleep during part of that 24 hour cycle, and be awake during another part. Large disruptions are worse than small ones. Waking up 1 hour earlier than normal occasionally is not a large disruption. Jet lag because you woke up 5 hours earlier would be a big disruption. What does a big disruption look like in terms of measurements? All of the many processes than normally oscillate together in phase go out of phase. I'm no expert in this stuff, but if you want to read about it, search for "chronobiology".