Nothing, except a GM who is unwilling to let it happen (random encounters, interruptions and so forth).
The fact that they're "per day" and not "per 8-hour-rest-period."
EDIT: Compare the text for the ring of sustenance (bold for emphasis):
This ring continually provides its wearer with life-sustaining nourishment. The ring also refreshes the body and mind, so that its wearer needs only sleep 2 hours per day to gain the benefit of 8 hours of sleep. This allows a spellcaster that requires rest to prepare spells to do so after only 2 hours, but this does not allow a spellcaster to prepare spells more than once per day. The ring must be worn for a full week before it begins to work. If it is removed, the owner must wear it for another week to reattune it to himself.
Right here is the main reason this doesn't work.
Recent Casting Limit/Rest Interruptions: 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.
If you cast one 3rd level and two 1st level spells in a fight then when you regain your slots your down one 3rd level and two 1st levels.
The sponteanous guys suffer from it too.
Recent Casting Limit: Any spells cast within the last 8 hours count against the sorcerer's or bard's daily limit.
And here it is from the divine section.
Recent Casting 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.
Divine spells don't require 8 hours of rest to prepare, but any spells cast in the last 8 hours can't be refilled. Also, if you don't prepare at the designated time (the time you choose to prepare your spells every day) or as soon as possible (if you can't do it normally) you can't prepare spells that day as a divine caster.
Ignore my earlier I had miss read what was going on though I don't know how you plan on counting time as rest if your up casting spell.
I'm a new player and I'm a bit confused about the spells per day. I understand it takes 8 hours of rest to be able to use those spells again. So what is to prevent a player from resting every 8 hours after every fight?
in addition to everything else posted, consider what that would mean.
Fighter: "Ok, we disposed of the guards on the entrance to the goblins' cave. Ready to go on?"
Mage: "No, we need to wait here for eight hours first."
Fighter: "Eight hours?!? You're useless! I'm going on alone, before they notice these ones are dead. "
You may require 8 hours sleep AND 1 hour to prepare your spells (doesn't happen on its own), but that's in addition to the "per day".
If you stay awake for 24 hours or more, you can't prepare spells because you haven't slept even though its a new day.
If you sleep for 24 hours (with 1 hour awake every 8) you can't prepare spells 3 times, since its still the same day.
The ring of sustenence just reduces the 8 hours to 2, but nothing else.
As others have said you can leave spell slots open and prepare them later that day when you have a better idea whats going on.
See here under Spell Selection and Preparation
