This answer on stackexchange seems to be claiming that you may prepare spells, using the extra preparation sessions during the day, to prepare spells on slots that were unusable due to the Recent Casting Limit, which says:
If a wizard has cast spells recently, the drain on his resources reduces his capacity to prepare new spells. When he prepares spells for the coming day, all the spells he has cast within the last 8 hours count against his daily limit.As with arcane spells, at the time of preparation any spells cast within the previous 8 hours count against the number of spells that can be prepared.
In other words, in rest for 4 hours, then you cast a spell at T, rest again until T+4 (so 8 hours have passed), but since you are still under the recent casting limit, one of your spells cannot be prepared. Then you rest again until T+8, sit down for 15 minutes and prepare a spell on that slot.
Is this correct? Is there any official source to back this up?
Recent Casting Limit/Rest Interruptions: If a wizard has cast spells recently, the drain on his resources r educes his capacity to prepare new spells. When he prepares spells for the coming day, all the spells he has cast within the last 8 hours count against his daily limit.
This is one of the generally ignored rules, I am told often even in PFS. It is extra work with little or no added fun for most. Even this post shows they are ignoring the immediately preceding rule that each sleep interruption adds 1 hour to the required sleep time.
Note that this thread ties in neatly with the current thread decrying sleeping in dungeons.
There is a trait that allows you to ignore one nightly interruption.
@Daw: That rules prevents an abuse. Without it, a caster could wake up 5 minutes before the 8 hours rest are finished, cast a bunch of long lasting spells, go back to rest for a few minutes, and regain all their spell slots. For prepared casters, this could up to double their spell slots when they have a day of rest beforehand, making especially divine casters pretty powerful. Now it at least takes a trait...
Understand the abuse situation, which is obvious for a GM to spot and deal with, just saying that in normal play people tend to ignore this. They tend to light touch the timekeeping.