I'm surprised to hear that that schedule worked well. Even if you woke feeling well-rested, what did you feel like towards the end of the day, at your normal/previous sleep time?
Personally in that situation I would (and do) get plenty of sleep every night and then skip the final night. I find the fatigue from a lot of lost sleep normally doesn't hit me in full until the second day after, and the final-day panic is enough to counteract the lack of sleep.
I’ve tried this because I’ve long thought my circadian cycle is just a bit longer than average, where I just don’t feel tired until late at night, and traveling westward feels great because I can take an extra long day. Waking up early on a regular schedule is hard for me generally, like, I can do it but I feel braindead. Eastward jet lag of 3-6 hours is awful if I have to do stuff in the mornings.
So I tried a complete “cycle” of 28 hour days until I realigned with the normal day. Which happens to be the same thing GP did. LCM(24, 28) = 168 which is 7 days with 6 cycles. Roughly 19 hours waking and 9 hours sleeping. I did let myself sleep longer instead of holding to 7-8 hours like GP.
It definitely felt weird because my wife wasn’t on the schedule, but I didn’t feel super sleep deprived. I’m sure with multiple complete cycles you’d see more adverse effects, so it’s probably best to only do this very sparingly. Maybe napping during the wake period could alleviate issues.
This sounds quite like Non 24 hr Sleep Wake Disorder.
https://en.wikipedia.org/wiki/Non-24-hour_sleep%E2%80%93wake...