Simply stated it’s an acquired jet lag. The physiologic basics for this lies in the fact that our brains are on a twenty six hour clock not a twenty four hour clock.
Let me illustrate. Say you sleep from eleven PM to seven AM seven days a week. You’re happy and have no sleep issues.
You get a call requesting your services in California. No problem you say, “I’ll just stay awake for two hours and sleep in for two hours and I’ll be on an 11 to 7 schedule California time”. Because our brains are on a twenty six hour clock it’s easy to “go west” up to two hours per night.
Three months go by and you’re called back to St. Louis. No problem you say, I’ll maintain my 11-7 schedule. So you lay down at 11:00 p.m. and your brain says “sorry Charlie” for me ifs 9:00 p.m.!
When you “sleep in” on the weekend your brain travels west. However, while we can easily stay awake two hours longer and sleep in two hours our brains can only fall asleep about 15 minutes earlier than our established sleep time. So for every hour we sleep in, it would take us four days to “get back home”. Since most of us only work five days per week we won’t get back home until Friday night. Then the cycle repeats itself if we stay awake late on Friday and sleep in on Saturday.
The way to avoid the delayed phase syndrome is to simply “never sleep in”! No matter how late you stay awake on Friday or Saturday night you must get up at the same time 7 days a week. This behavior leads to sleep deprivation so by Sunday night you will be sleepy enough to fall asleep quickly and never “travel west”, your brain will stay in St. Louis.