As a precursor, even if a stringent sleep schedule is absolutely followed, one still needs to practice other good sleep hygiene habits: avoiding blue light (or using a filter for screens), exercising regularly, eating healthy and attaining an overall equanimous mood (lack of distress). That, among other non-ideal factors, such as sleep disturbances could also result in a poor nights sleep. But the question was stated as if in a 'perfect vacuum.' Non-idealities are overlooked.
That being said, I am going off the aforementioned assumptions about religiously going to bed at 10:45 and waking at 6:30 every day--including weekends. Let's say this person could sleep for nine hours (9:15 TIB), and we can safely assume that an adequate nights sleep is 7:45 TIB. In other words, this person can be high-functioning on 7.5 hours of sleep (five phases).
→ Is it better to wait until 12:15? (Either way, 6:30 wakeup for work stays constant.) One deviant day--does it matter? When will the sleep debt be relinquished? Honestly, I didn't find any research remotely about it.
Let's go out on a limb, and say it doesn't matter given the ideal-vacuum conditions. Then how many deviations, say, twice a week--do matter? How many will lead to partial sleep deprivation? Still, the answer is unknown.
So I emailed sleep researchers from U.W. Madison and Stanford:
Going to sleep in the middle of a phase versus "catching" the next phase (with less sleep) for one night (subject)
Hello Professor _______,
I have question for you about making a bedtime choice, a deviation,
from a consistent sleep schedule: For an average adult, in general, if
one knows they will wake up in the middle of a cycle--is it better to
go to bed later?
Get home late, and ready to be in bed at 11:15 PM. Must wake at 6:30
Get home late, later, and ready to be in bed at 11:45 PM. Must wake at
6:30 for work.
The first is 30 minutes late. The second is 60. Let's say for this
person that we can safely assume that an adequate nights sleep is 7:45
TIB. In other words, this person can be high-functioning on exactly
7.5 hours of sleep (while following a consistent sleep/wake routine).
→ Is it better to wait until 12:15?
[...] I ask this question because I follow the routine above. I
deviate 1-2 times a month, and either way usually take a nap before
3PM to follow, but persistently wonder which is the ideal choice.
There is not easy answer, I do not know of direct evidence
for one schedule over the other; you are the best judge; try the
different schedules and decide which one works for you (i.e. you feel
better and you are “ready” to go)
It may be that they are very
similar; in any case, it is dangerous to assume 7 or 8 hours is what
Inter individual variability is huge, this is why each
person is the best judge
Unfortunately, there is no good answer. The data for waking up in the
middle of a cycle is sparse and doesn't offer good conclusions (the
only reasonable one is that if it makes you feel better to wake up at
the end of a cycle, that's probably a good thing). There are some
circadian consequences of bouncing around a sleep onset time too much,
with greater variability in onset time leading to more difficulty
initiating sleep (and probably some other changes as well, though less
well documented). Me, I'd stick with the earlier bed time that is
closer to standard bedtime and get the extra sleep...
Assess your ideal amount of sleep by taking a 'sleep vacation.' Over a two-week period, keep your consistent bedtime, but don't wake up with an alarm clock. Log. (See healthysleep.med.harvard.edu/need-sleep/what-can-you-do/assess-needs.)
How do you objectively analyze your sleep?
There are two commonly used methods to assess sleep, Actigraphy
and Polysomnography. Actigraphy is a non-invasive method of
monitoring sleep and involves wearing wrist activity monitors, which
are devices worn like a wristwatch that continuously record body
movement (usually stored in 1-min epochs). [...] Actigraphy is useful for
understanding sleep patterns as it is non-invasive and relatively
easy to collect data over significant periods of time (commonly two
weeks of monitoring).
The second method is Polysomnography (PSG) which is a sleep
study in which body functions such as EEG, eye movements,
muscle activity and cardiac activity are measured. PSG provides
information on sleep staging and is considered the “gold standard”
for assessing sleep quality and quantity.
~ Sports Science Exchange (2013) Vol. 26, No. 113, page 2
→ DIY: https://sleepjunkies.com/features/the-ultimate-guide-to-sleep-tracking